Greg at Antivenom is in Sefner, so he is closer. Were in Bradenton.
Since your vette has an HEI dist, (and not the dreaded OptiSpark) all could be from the chip. Tuning your generation of vette with a piggyback chip is guess work at best & only a custom burn't chip based on a dyno session & drive will be right. Anything packaged is a "one size fits all" and rarely is ideal.
I'd say with what you have described so far the shop doing the test's is going in the right direction as they have done everything I would have, except the chip.
Look in the sponsor section for the link to Greg's shop & the other site sponsors.
I really think your close, and I do not think it is the heads at this point.....your going to end up with a nice running car I predict.