From personal experience, I'd say go with a GTP motor if you can. Don't go with a Riviera motor. I'm using a '98 Riviera motor, and attached 4T65E-HD auto tranny. Turns out the '98,'99 Park ave and Riviera used a slightly different 4T65E with a different sized pass. axle, which was a pain to get. Also, you need a GTP (or other) oil filter mount instead of the Buick's. And the Riviera's (and I assume Park Ave's) SC tensioner / idler pulley setup puts the belts right into the passenger side hinge, not a good thing. You need to design a new pulley setup or use the GTP pullies. The computer seems OK to me, but I haven't *really* looked into it. People say the Buick antitheft Passkey is harder to deal with, but I found it easy. Just use the original Passkey box from the car. Just some thoughts from my experience! (and BTW if I had to to it over again, I'd go 5 speed Getrag!! Axles were a pain!!)
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Bob Williams
Multi-colored '87 Mutt, a work in progress! (3800SC installation in super-slow-motion progress... I turn the key and it starts, I turn the key off and it stops. I give it gas and the axles spin. WOOHOO!! It works!! It works!! It works!! Did I mention IT WORKS!?!?)