Heads

All 3800's were equipped with Iron cylinder heads, however there are a few differences between the series 2 and series 3 heads as far as performance goes.

Series 3 and series 2 heads are identical and can be interchanged, however the series 3 heads have slightly larger intake valves.

Series 3 intake valves measure 1.83“ in diameter

Series 2 intake valves measure 1.80” in diameter

https://www.gmforum.com/performance-brainstorming-tuning-96/sliced-2261-early-l36-castings-4781-castings-next-253593/page2/

Stock heads will support .525“ lift on the intake and .528” on the exhaust valves before running into risk of contacting the valve seat.

http://www.fullthrottlev6.com/forums/showthread.php?56830-max-valve-lift

You can of course machine the valve seats to allow more lift. Another option is to utilize viton valve seals and modified retainers to make a little more room for more lift.

Stock heads can be ported quite a bit to see a lot of gains. Most aftermarket heads are just precision ported and polished factory heads.
Check out the porting and polishing page for heads: Porting and Polishing Heads