the stock L98 heads really arent that good, the aluminum ones are better but still dont flow that well. the heads however are not the bottle neck if your running a stock TPI, its the lower intake manifold. they changed the intake bolt pattern from straight to angled in 87 so you will have to use a head from that time period to avoid modifying the lower manifold which can be done and the article below explains that a little, i have some heads off of an 88 that are cast iron if your looking for a set. i assume your running the TPI and that is why you need the heads?
here is a little article on it
http://www.hotrod.com/techarticles/engine/hrdp_9812_tpi_engine_swap_for_classic_chevys/length_of_the_intake_runners.html