1977 Other Oldsmobile Models sarting issue

Engine Mechanical problem
1977 Other Oldsmobile Models V8 Two Wheel Drive Automatic

I recently replaced an ailing 403 Olds engine with a reman long block. I finished all the details and yesterday I was ready to start the car. I get backfire if I throttle the carb, but I get no response. I checked that I had spark and do...I reset the rotor on ignition to make sure it was set...but I still don't get a start...not really even a hint that it wants to fire except for the backfire i get if I give it gas...?? What is left to check? Could the intake be leaking by and allowing exhaust back into intake port? This is all on a 1946 street cruiser that I want to get back into circulation...