Help... Starting Problems, Rough Idle, and Backfire through Carburetor

[Green Monster]

I have a 72 Blazer with a 350. The truck was originally from California and is now in Pennsylvania. I never adjusted anything on it since I have owned it because it always ran great. The motor was replaced with a crate motor around 2000 (year). The truck ran great and sounded great since I have owned it in 2003. However, two weeks ago the engine would stumble and the carb started to backfire only at high RPM’s under heavy acceleration. I though it just might be the colder weather causing this. I tried 92 octane gas and this did not make any difference. I usually run 87 octane gas. I don’t really know much about carbureted vehicles. This is my first. This seemed like minor issue.

Yesterday, I filled up with 87 octane as I usually do. Drove three miles. Parked it. Came back out five minutes later and the engine would not start. It would crank but not start. I checked for spark and it had spark. I pulled the distributer and check to see if it had points that needed to be adjusted. It does not have points. I check the dwell while cranking l and it was at 30. I finally got it to start by giving it pushing the gas pedal as I cranked. But when it started it idled very low. I drove it home one mile and it stalled in the driveway. I started it again and the carb backfired when I stabbed the gas. It will start but misfires and stumbles. I need depress the accelerator to keep it running. The RPMs run up to 2,500 then drop like it’s going to stall, I give it a little gas and the RPMs race up to 2,500 then drop like it’s going to stall.

What could be the cause and how do I fix it? Any suggestions?