All of the 60 degree V6s from GM use the same heads. That is, 2.8, 3.1 and 3.4 heads are 100% interchangeable within the same design. The iron heads have a larger chamber, but flow quite well for their era. We build them "up" with larger valves and better springs. The only similarity I can see to the old Buick would be the "zero" valve angle. The ports and chambers were "current" in 1980 when the engine was first released.
We've done many of these engines for both S-10s (and 15s) and Fieros. The iron-headed engine can make 250 HP without MAJOR work. Porting, cam and a good balance, along with larger induction will do the "trick". Edelbrock still offers an intake for them, using either the Rochester that came on them, or with an adapter, a small 4-bbl.
The aluminum heads (the splayed-valve units you speak of) are VERY good heads. If you can move the entire intake assembly "over", and get the injection wired right, you'll realize significant power gains. The water pump drive and plenum may take some creative "enginieering" to make them actually "work". The intake, etc. from the aluminum heads wll NOT fit the iron heads.
Jim