1987 Other Chevrolet Models Back firing running rough and lo

Engine Mechanical problem
1987 Other Chevrolet Models V8

I have a ski boat with a 350 chev motor with marine conversion. It has just been completely rebuilt with new pistons, rings bearings etc, new marine cam and timing gear set and new heads. It had an old distributor which was casig problems so changed to a HEI electronic distributor, a procomp 6000. New leads plugs etc. I tried it out yesterday and it ran very sweetly to start with. It had full power, no backfires or missing. It ran like this for about an hour and then started to idle rough at low revs but was still excellent at higher revs. After using it for about 2 hours, its was now getting very hesitant to accelerate often backfiring. As it backfires more, I appeared to be losing power in the motor. It is now running ok at idle but no acceleration when asked of it and seems down about 50-90hp. It has a holley 780 double pumper on it which I eventually want to change to a 600 but cant explain why it was running sweet to start with but so terrible by the end of the day.