carb backfire edelbrock 1406

[sagacious]

i have a 73 nova with a 350 stock engine, i've replaced the lifters with performance type, and added a double roller timing chain that is advanced 4 deg. the ignition is aftermarket HEI with 55,000V coil, and MSD ignition module. the intake is an edelbrock air gap rpm and the carb is edelbrock #1406 (600cfm). flowtech headers and 2.5'' dual exhaust straight back, 4 aluminum radiator and 160 thermo.. the engine runs great and smooth until i hit higher rpms.. then the carb starts to backfire a couple times and you can tell hp drops a little...
what could be causing it to backfire? the car seems to run better the more the timing is advanced. and it is advanced all the way right now, but the problem is still there. i've noticed a small difference when the car is running hotter... it seems to get "better" than it was.. but never goes away. i just fixed the ac in the ac and i've seen the temp hit 210 at idle for a while and it'll run 190 when cruising. that seems a little outrageous to me and too hot.. but like it said i've noticed it's not AS bad when its hotter... or it could just be me. it's a low compression 8:5, and without the ac on the car stays 160-170 temp. could too much fuel be going into the secondaries? too little air? too large of jets for the stock engine? any advice is appreciated.. thanks a lot.