My first thought was compression as well. Was there a difference in the size of the combustion chamber with the new heads? The valves not being adjusted properly is a good thought too, except that it would work the other way: if the valves were too tight, they wouldn't seal, and you'd have low compression. The car would take a long time to start, but would crank over quickly. If your valves aren't opening enough, it could create the problem you're having. It could also be caused by higher than normal resistance between the relay and the starter. You should do a voltage drop there and see before anything else.
Take care
~Chris
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