Engine Performance problem
1979 Chevy Blazer V8 All Wheel Drive Automatic 0

I have a 79 blazer, that i just preformed an engine swap on. The engeine is a 88' tbi converted to carb. It has stock 1.92 cylinder heads, 10:1 pistons overbored .040, long tube headers edlebrock preformer intake with the stock 79' q jet carb on top. When the engine is reved it back fires between mid and high rpm. does there need to be jetting changes the the carburater? it worked fine on the old bone stock 350. Or could the timing be off? I have it set at the spec in the service manual, and if i rotate the distributer aew degrees either way the running doesnt inprove. what do u think? or do you have any tips?