Can't figure what's going on with my truck

[Cochise]

1970 Chevy C-10 with 350/350 out of a '72 Cheyenne. I was having trouble starting my truck back after it ran for awhile. My mechanic who works at the dealership I work walked me through the motor install and told me it was either the starter or the HEI distributor shaft may be stuck. I took the wires off the cap, looked below the rotor bug and decided since it moved freely that it was the starter. After I replaced the starter I put everything back the way I had it and would turn over fast, but wouldn't start. I pulled the cap back off and noticed that I pointed the rotor at number 1, but for points, so I went to TDC, raised dist shaft and pointed at number 1 for HEI, put it all together and I either get a huge backfire (at midnight) or I get fire through carb. I have checked the firing order 8 times to be sure and clockwise.
1. I am not positive that the 3 pin wire on the Dist is plugged all the way in, could that be?
2. What else could it be?
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