exhaust pops
 

I have a 1968 GTO with a 461 stroker that was running strong last fall. Over the winter I pulled the heads, Eldebrock aluminum RPM's, to do some machine work.
I put it all back together and now I am getting a pop out of the exhaust when I back out of the throttle. The idle is a little rougher than it used to be also. I checked the ignition completely which is good. I replaced the power valve also.
The gas in the tank was from last year but had Stabil in it. I also put gas in it with octane booster. The local gas station doesnt have race fuel yet.
I am wondering if the intake gaskets did not seal properly but i've never had a problem with that before.
Is there somewhere else I should be looking or a definitive test I should do to localize this problem?

Frustrating!!!

Any way your timing could be out?