I agree with SirDan, in your circumstances I don't think it would be worth it to go through the trouble of swapping heads just to get back some MPGs. Tune it up the best you can and have fun with it now and in the meantime do research on building a proper engine from the "block"-up. Also research how to tune Holley carburetors and have at it; when it comes to classic muscle cars and hot rods 90% of the time you have to do it yourself if you want it done right.
IMO the only "right" way to fix your engine is to swap the pistons and zero-deck the block to raise your compression. Any other method is more of a band-aid fix and will be more trouble than it's worth, esp. if you are satisfied with the performance the way it is now.
