Carpet kits just don't fit right. Get an upholstery shop to custom fit the carpet. Not sure on the rest.
Never used opgi for seat upholstery, be more inclined to buy the material and have a custom shop install it, probably will put in new foam also. As far as the carpet goes if it isn't really hammered take it out and have it professionally cleaned. Then have it dyed, you can match it to a side kick. There are people out there that do this. We have a guy in Ct that does this and vinyl repair and he is the best. I did this to the carpet in my 64 and it looks great and stays original. Besides most of the carpet from these places just isn't that great, but then it depends how fussy you are.