After reading all your posts I bet you have the timing right and your rocker adjustments are too tight. I just got done putting a crate in my '69 Camaro and I had exactly the same problems that you are having. Crank and crank, really smoothly cranking too then an occaisional backfire through the carb.

I'm certainly not the one to listen too closely to when talking about valve adjustments. I thought that taking the play out was taking all the play out. I couldn't even rotate the pushrod.

What I finally did to get it right is to loosen them up till there was a little bit of play while I moved the pushrod up and down. Then I gave them another 1/4 turn. It's kind of like adjusting them when the engine is running, back the adjustment out till you hear them clicking, then tighten them down a 1/4 turn or which is proper.

After I got them right the engine fired right up.