454 backfire

Last night I was helping my brother put a reman 454 in his 77 one ton tow rig and when we got it started we have a backfire threw the exhaust on one cylinder. We went over the wiring two or three times and it’s not that, it seems to do it more when it’s cold. After we warmed it up it went away but later he called me and when it cooled down and he started it again it was backfiring again. Any ideas??

Thanks for any help.

put a vacuum gauge on the engine see what the gauge reads when it backfires...it you have rapid movement of the gauge when it backfires you have a sticking exhaust valve