I have a 1985 Chevy Scottsdale half ton and I just put a new 454 in it and everything is brand new. It runs great when it's cold but when it's been running for an hour or so it starts backfiring and misses real bad when you get on it a little bit but once u turn it off for about 45 min it runs fine until it gets hot again and starts it all over again I've change spark plugs plug wires put 2 carburetors on it new distributor I'm not for sure what's wrong with it now??