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
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