i have a 68 350 and i turns over but wont start and backfires through the exaust

 

Can be a number of things. Exhaust valve sticking open so it's lighting off through the exhaust, timing is off, other ignition or cam related problem. You didn't mention if you have aftermarket CDI ignition or such or what carb etc you are running. Hard to get a good idea when we don't know how many miles are on it, etc.