Backfire........need advice - please help

[69 air cab]

I just removed the 305 engine from my 84 Silverado and replaced with a new 350. Had the carb. professionally rebuilt, installed new compoents in the top end of the distributor. Tried to crank it and it backfires only.........will not start. Even caught fire for a sec.

I believe I have the spark plugs wires routed correctly.
Carb should not be the problem.
I may have some vac lines incorrectly routed - could that be it?
When I installed the dist. I found top dead center of Number 1 plug, marked the dist and faced the mark toward the #1.
I have tried turning the dist a bit to see if it stops backfiring.

Can you tell me what some of the things are that I need to check? I'm into my third afternoon (after work) and ready to call a professional.