Please help me!- wit's end.

[SeanPaintsFlames]

I really hope somebody can help me out and point me in the right direction. I've got this 65 GMC. It's a piece of junk. But, I've been trying to successfully rebuild the 250 straight six in it. After taking everything but the shortblock apart and replacing or rebuilding it all, it was running okay other than a high idle and some off-idle hesitation. So, to fix the idle issues I replaced the carb with a remanufactured one, and now it has a really really (really!) wicked backfire out the exhaust (or after-firing, so I've heard). Now, I know that cherry bombs are known for that engine-braking pop, but this is retardedly stupid. I have the split headers collected into a pair of cherry bombs and that's it.
If I'm not WOT, it's backfiring. I'm talking fire-out-the- exhaust popping. If I try to maintain a constant speed, it backfires on what sounds like all cylinders until I either let off the gas or put it to the floor. I've tried to adjust the timing, adjust the air/fuel, and no difference. I picked up a timing light today, and when I hooked it up, the timing mark on the crank pulley wasn't even close to the metal tab with the marks on it. It's weird 'cause I've put a good thousand miles on it since the tear down with nothing like this prob, but as soon as I get the new carb, this starts.
So you know, its a 250 Inline six with headers, HEI, and a single barrel carburetor.

Any thoughts?
Thanks so much guys!!