i just recently installed headers on the truck having trouble with timing/carb tuning it has a 2 barrel carb also a single vacuum distributor also recently replaced wires plugs rotor and cap the truck will idle but when placed in gear it backfires thru the carb and thru the exhaust . the truck has original three on the tree any help will be appreciated..

 

If you don't have a timing light try to borrow one. It being a std tranny putting it in gear has to have little to do with the equation. It must be the acceleration of the engine under load. Does sound timing related but could be a few things. use the KISS method (keep, it, stupid, simple) I'm guessing the engine didnt do this before the headers ? You didn't move the distributor for any reason installing the headers ? Maybe a vac line has been left loose somewhere. I'd look around for a vac leak if the timing checks out. 6-10º BTDC at 650rpm in (N) with a stick shift IIRC
Good timing/no vac leak, maybe a valve sticking open or other.. keep it "....." dont start wrenching all over before having a look at each attempt's success rate.

 

Generally speaking backfiring thru the carb is indicative of a lean condition and backfiring thru the exhaust a rich condition, as redmanbob mentioned get your timing set first and for most and then make any carb adjustments if need be.