heads
The best gains come from getting your stock ones ported. You can either buy a used set or send yours in.
You can get more out of some LT4 or aftermarket heads, but the cost is a good bit more and you won't see a lot more from those heads if you get your stockers done right.
There are several companies that port LT1 heads very well and there are PLENTY of places that port them very poorly.

