Common Sense + Critical Thinking
I assume that you have shopped around for heads as well, there are a bunch of options there as well, even completely upgraded sets of heads can be found for a good deal. You can get aluminum, or stay with cast iron, here is an example: NEW 2 GM CHEVY VORTEC 350 5.7 CYLINDER HEADSthe basic parameters of the engine are set: ported heads, upsized stainless valves, mild springs, roller rockers, keeping pressed studs, mild cam.
Jegs and Summit racing carry a bunch of options. Some new heads might be cost competitive with rebuilding, just something to keep in mind.