Ok here goes I have a mild built sbc in an 1981 Chev 1 ton chassis and a custom 1950 cab mounted to it as a work truck. Been a few year build constantly upgrading but here is where I am at now.
1981 sbc stock bore stock crank 16000original miles rebuilt it because as everyone knows bad year for camshaft.
Put new rings bearings honed cylinders
comp cam110 degree normal lift longer duration cam
Double roller chain stock pin on cam gear
New jegs stock length pushrods
New roller rocker stock ratio valve train
New jegs 2.02 aluminum heads
New champion plugs
New spark wires
new hei distributor
New680 quick fuel vacuum second
Jegs duel plain high rise intake
All brand new gaskets
Custom 1 5/8 right to 3 inch no restrictions stack mufflers
Turbo 400 trans
4.10 rear gear

Timing is set to 12 degrees
Carb has 11 inches of murcury max at 600rpm idel
Jetted the carb from quick fuel had it at 70 78 I have it down to 68 72 and the plugs are still black and some wet. Number 5 cylinder is just starting to turn dark brown.
Vacuum ported advance
6psi fuel pressure
Fuel is in middle of the sight glass on the bowls

Now here is the problem the truck is a total dog has no get up and go. And I have a dead spot from 2000-2500rpm where it backfires out the exhaust almost constantly and even will ocationaly stall and refire under light accelerations at slow speeds. It seams to run fine before 2000rpm and after 2500 it stops back fireing but feels as if it had a miss in a random cylinder every once in a while.

Should I be looking at the weights in the distributor or fuel delivery system. At a loss everything I try dose not seam to remidy the problems.