72 C20 402 big block backfiring

[project_sinister_c20]

Well i have a 402 big block mainly stock, performer rpm intake,demon carb,header, dual exhaust and just swapped it from points to a streetfire hei and from a edelbrock to the demon 750.

from what the previous owner tells me its a stock cam but it doesn't sound stock, he also showed me pics of him going through the motor to freshen it up but i doubt his abilities after the rear end he "rebuilt" blew up on me.

any who ive been chasing my timing around and cant seem to get it right and could use some help.

Iv'e started from scratch on the timing going from 8* to 16* and still get backfire through the carb and it just falls on its face. the vacuum advance doesnt seem to right either because the total timing doesn't seem to want to drop below 40-45* at around 2800rpm.

any help will be greatly appreciated!