75 318 Dart won't start and backfires!!!

This my first "project" and I've run into a snag. I bought a little '75 Dart with an original 318 4bbl w/ 49000 original miles and have swapped the intake manifold and carb. I put a Weiand intake and Holley 4160-1850s 600cfm carb in. I've tried to restart it and it seems like it gets no spark, then about 2 seconds after you let off the key it backfires! It has drove me nuts for the last couple of days and I don't want to tear anything up internally by not knowing where to begin. ANY help would be greatly appreciated, THANKS to anyone with ideas.