I am copying this information over from the old 63/64 site. In time I will continue to add to and edit as needed.
1963 390 and 1964 429 Engine History and Differences
-The 1963 390 was a totally redesigned engine compared to the 62 and earlier 390's. It was 50+ lbs lighter, engine accessories including the distributor was moved to the front and used a new aluminum engine cover that incorporated the oil pump. Nearly 1/2" was shaved off the top of the block, connecting rods were shortened to keep the same compression ratio, block length was shortened by more than an inch. They switched to a hollow-cast crank, which is 11 pounds lighter than the 1962 crankshaft, and switched to an alternator-rectifier charging system, which saved an another 10 lbs over the generator used in 1962. The bore, stroke and horsepower (325) were the same as the 62 390 however this was a lighter, more agile 390 than the old design.
- For 1964 they took the new redesigned 63 390 and increased the bore and stroke increasing the displacement to 429 cubic inches and horsepower to 340.
-They also used two different crankshafts on 64 429's (see Transmission section below)
- The 1964 429 used with Hydramatics had a crank with a raised rear "drive plate" mounting area that was 3 3/16" across.
- The 1964 429 used with Turbo Hydramatics had a crank with a raise rear "flexplate" mounting area that was 3" across.
-Other than a few other minor changes including redesigned right exhaust manifolds, larger higher capacity oil pump gears in the front aluminum engine cover, air breather housing inlet enlarged, power steering pump cooler added, etc.) the 1963 390 is very similar to the 1964 429....especially the 1964 429 with the Hydramatic crank...as they will both mount up to the older style Hydramatics.
-In 1965, the engine was moved forward, the motor mounts were changed, the oil pan was changed to a front sump pan and their were other changes such as pistons that are different from 1964.
- In 1963 all body styles came with the Four Speed Hydramatic (shift pattern PND1D2LR and sometimes called Jetaways) transmission and bolted directly to the back of the 1963 390 block.
- The Hydramatic transmission between 1961 to 1964 will interchange with no modifications. They changed the reverse cone in 1962, but the transmission housing itself is the same.
- The 75 series and Commercial Chassis Hydramatic were slightly different in that they had a heavy duty reverse clutch and higher capacity oil pump, however, they are externally physically the same and will bolt right into a regular body style. (I put one in a 1964 62 series coupe some time back.)
- In 1964 the 62 series, 75 series (limos) and commercial Chassis continued to use the Hydra-matic and again bolted directly up to the back of the 1964 429 block.
Turbo Hydra-Matic Transmissions
-The new 3 speed Turbo Hydra-matic was introduced in the Buick as the Super Turbine 400 in 1964. In order to not let Buick get a jump on Cadillac, The 1964 deVilles, Eldorados and Fleetwood 60 Specials used the Buick transmission using an adaptor ring to mount to back of the 429 engine. It was then introduced in Chevrolet, Pontiac and Oldsmobile in 1965.
- The Turbo Hydra-matic used in 1964 Cadillacs had a single range Drive shift pattern - PRNDL and is often called a Turbo Hydramatic 400, THM400, or Turbo 400 since other makes (e.g. Buick Super Turbine 400, Chevrolet Turbo Hydramatic 400) used 400 in the name. It has also become the common name used to refer to this family of Turbo Hydramatic transmissions.
- The 1964 "Buick Borrowed" Turbo Hydra-matic was a one year only transmission for Cadillac. Cadillac got its own new Turbo Hydramatic transmission in 1965. For 1965, the bell housing casing was changed to allow the transmission to mount directly up to the 65 429, and the internal hydraulic circuitry was modified to provide a dual drive range as seen in the 1965 Cadillac Shift Quadrant below.
- Swapping the Transmissions ---> Things to Consider
Hydramatic Drive Plate with 3 3/16" opening & Turbo Hydramatic Flexplate with 3" Opening!!
==> While the hydramatic drive plate with the 3 3/16" center opening will mount right up to Turbo Hydarmatic crank since the bolt patterns are the same, there is a gap and will usually rip out in short order.
==> the Turbo Hydramatic flex plate opening has to be enlarged to exactly 3 3/16" to mount to the Hydramatic crank end and...if it is not exactly centered and a snug fit it to will quickly rip out.
==> In addition, the inside diameter of Hydramatic crank rear center is 1.6", while the inside diameter of Turbo Hydramatic is 1.7". This may cause the front center cone section of the Turbo Hydramatic torque converter to not have enough clearance, requiring either the rear center section of the Hydramatic crank to be honed out at least .1" or the nub of the Turbo Hydramatic torque converter to be honed down in order to mount the Turbo Hydramatic torque converter on the back of the Hydramatic crankshaft.
===> A best option is the swap out the correct 429 crank designed to fit the Hydramatic drive plate or Turbo Hydramatic flexplate.
To go from Hydramatic to Turbo Hydramatic or vice versa consider these additional differences between cars with the Hydramatic and the Turbo Hydramitic
==> Front driveshaft section on TH equipped cars is longer (putting a TH in a 63 or 64 limos requires the front drive shaft section from a 1965 limo)
==> Front & Rear cross member different 63 to 64 & Yaw Mounts and the rear transmission mount are different
==> Starters were different
==> 1964 Turbo Hydramatic has adaptor ring, hydramatic doesn't
==> TH uses electric kick-down switch, Hydramatic used the TV Rod
==> carbs: The original Carter AFB and Rochester 4GC carbs that came with the Hydramatic equipped cars, will not have the mounting block for the THM trans kickdown switch, so you will need a carb from a THM equipped car.
==> Instrument cluster shift indicator display (PRNDL on TH and PND1D2LR on Hydramatic.
==> Hydramatic uses a drive plate that mounts to the torus while Turbo Hydramatic has flexplate with starter gear teeth that mounts to the torque converter
==> turbo Hydrmatic had vacuum line from vacuum modulator up to vacuum fitting on back of carb where brake power booster connects
==> Neutral Safety switch is different if swap out is in a 64, due to different in shift pattern
==> back of engine transmission mounting dowels 1/2" for Turbo Hydramatic equipped transmission and 5/8" for Hydramatic equipped transmission. Dowels need to be swapped out when making a transmission change.