Okay here we fall back on modding the TBI...
If you do run a differant manifold, you are gonna run in distributor clearance problems. Go pop the hood on your 91, look where the windsheild cowl hangs out over the distributor. 91-93 B-bodies use a specific intake and distributor. The shaft on the distributor is shorter and the hole for it in the intake is wider. This allows one to remove the distributor without pulling the intake manifold... because the windshield cowl would otherwise block it.
You can run a typical "standard" SBC intake (assuming you get one that will bolt up to the 87 - 9? intake bolt pattern) but you will have to use a non-B body distributor and wont be able to pull it without pulling the intake. The stock intake is not that bad!
Spend the money you would on the intake on exhaust. Even if its just the later Y-pipe. Get 1.6:1 stamped metal rockers (about $100-120), use new LT1 style valve springs and lose the rotators on the exhaust valve (removes weight from the valve train), do all the maintanance things, rebuild the throttle body. Get a caddilac TBI airbox and build you a good air intake with PVC pipe, ditch the stock belt driven fan for an electric.
for info on enhancing the performance of a TBI engine. Here is a hint... you cant go far without burning chips or spending money on special ECM mods.