440 Won't start syndrome

Sounds like it could be either the timing chain or the distributor the way it blowing back thru the carb. My experience has been that even with a huge vacuum leak you wouldn't get the blowing back thru the carb like that. You will get pressure on the intake and exhaust side, if you want to be positive use something to go thru the spark plug hole and feel the piston and check your wiriing after that. I agree with Lonewolf it definetly souns like timing or possibly dwell. Try setting the points with a feeler gauge and then set the dwell. Good Luck