n20racer,
That is a nice BBC you have there. In most cases if a car will only see the strip once a month I would usually recommend the Turbo 400. But it depends on what your goals are. If you want 1/4 mile ET's I would go with the P/G trans and that's because your BBC will keep the car in it's powerband from a 1-2 shift and the 4.30 gear is enough where you don't run out of engine with say a 5.13 gear or steeper gear for launching, especially if a 5.13 gear is behind a 350 SBC making 400 HP
However, that being said the Turbo 400 is a stout unit, it is cheap and it makes a better street trans than the Powerglide. I would go with the turbo 400. :mrgreen:
I have 4.10 gears in my car now and let me tell you it $*cks over 50-55 MPH. The problem is the GM overdrives other than the 4L80E will have a hard time handling the power you are making.
Since this is more of a street trans, that stall steep would be crazy high wih a Powerglide and you still could run of out of RPM.
Just my 2 cents.
P.S. check out
www.turbobuick.com Look at the cars on their home page on the left side. There are five 1980's Buick GN's/T-Types. All five run turbo 400's. Either way is fine, but for your application I like the Turbo 400.
P.P.S. On my first 1969 Camaro, that car had a P/G from the factory. Nice trans! But eventually it was replaced with? A THM400
Good Luck!!