Speaking from experience here..
(Had a Buick 455, in my 73 Electra 225)
JW has great advice on the upgraded ignition. Either Mallory or MSD make a good drop in HEI. I went with a pirated HEI from a 74 455, with an MSD controller and upgraded cap, rotor, etc.
I had an Edelbrock 750 carb that, with some tuning, really woke it up. Ditched the factory quadrajet and saw a much improved torque curve and peak HP.
I also recommend TA Performance headers. They really let the engine breathe, and it was no longer running out of steam on the top end. I did a TA Performance header (chrome of course) through true duals into a glasspack on each side, then turn downs after. It sounded so sweet like that, but those headers made a big difference in how the engine behaved. It took away a bit of off the line torque to give it some serious mid and upper HP. Pricey, but very worth it. TA really has a lot to offer when it comes to the Buick 350s and 455s.
The 3500 stall is going to be way too high for that motor. You probably want to keep it around 2500.
The problem with Buick motors, is so few people make parts for them. You're gonna spend a lot more to make power out of one than a Chevy. Summit has some parts, but not a lot. TA Performance and Buick specific vendors (Jack Cotton'* Performance in Agawam, Mass) are more likely to be able to help you out, and know the motors a little bit better.
2004 Buick LeSabre Custom FE5 GXP suspension conversion. 170Kmi
2001 Silverado 4WD 5.3L, 33" BFG ATs, Z71/Z82 pkg, 147kmi.