If it's backfiring through the carburetor, you've either got one or more intake valves adjusted too tight, or your firing order is wrong. Take the valve cover off, and turn the engine clockwise from the front until #6 exhaust valve is closing and the intake is starting to open. The distributor rotor should be pointing to the #1 cylinder plug wire at this time. If not, rearrange the wires so the rotor points to #1, followed by 5-3-6-2-4 in a clockwise direction. Setting the timing with the electronic setup will be a little tricky, but you should be in the ballpark close enough to start by moving the distributor a few degrees right or left while cranking. It's much easier to set exact timing with a set of points or a GM HEI setup. I'm not familiar with the triggering process on the aftermarket HEI units.
Once the engine starts, confirm that you've got oil pressure and bring the speed up to 2,000 RPM or so for 20 minutes. It's OK to tinker with the timing to get the smoothest run, but don't worry about an exact setting at this time. Fooling around with timing and idle settings before the cam and lifters break in can result in a scuffed cam and rapid wear. The cam and lifters need the splash-oiling that increase crankshaft speed provides during the initial wear-in procedure. Unless the engine tries to overheat, don't idle it for at least 20 minutes after initial startup. Once the cam break-in is done, idle it down, shut it off, retorque the cylinder head, and readjust the valves. Now it's time to restart, and deal with final timing settings and idle adjustments. This procedure has worked for me for about 40 years, even on some very high-dollar round track race engines.
Jerry
