well i just finished putting on some dart iron eagles (180cc,64cc) and i finally got it all together and it backfires now when i let off from high rpm like say after a 1/4 mile run(its an auto) and when it does it at night i can see the flame, the motor is a 350 with 274 cam rpm intake and those heads

is this something i should worry about...i have the timing advanced to 8 degrees, but there is also a bad exhaust leak because i cant get the last header bolt in on the back of the passenger side header, or could it be a valve setting problem?

 

I wouldn't worry much about it. It can be embarrasing though. Probably not a valve setting issue. The different combustion chamber shape probably wants a different timing curve.
You may want to go leaner on your idle mixture and lower on the idle RPM setting.
Uneven fuel distribution with one cylinder too rich can cause this also.