I bet the throttle valve cable isn't set correctly on the 200
as for the th400 , it would be hard to kill a well built one in a 3500 pound car, and its half the price of a 2004r
Many dont agree with this but i run Vr1 20/50 in NY summer and 10/40 in the fall/winter for 20 years , with only showing a 3 psi hot idle difference
A front mount will put you over 200 degrees on the streets without good dual fans
Valve body, governor, speedo gear is different, its calibrated for a 305 and .373 gears, it can be made to work but wont handle much more power then a stock Turbo Buick has with out being built.
Definitely pick up an inexpensive Harbor Freight laser temp pointer they are pretty accurate , i point one on the blocks and intakes and intercoolers/pipes sometimes, never pointed it on the dp though
Your not going to be able to manually shift any H pattern as fast as these cars spool up and go, 1000 fwhp and under go for a 200-4r by Husek
As cool as a T-56 is, technically its only gonna slow you down in these cars