Hello, I have a mustang 1967, I install a 302 engine from a 1974 mustang, with a c4 tranny from a mustang II and a 8" differential. I just finish all work on my car, and when I drive it, it sound good (flowmaster super44 with 2.5" tube). you feel the power of the engine, but when you hit the gas, the car doesnt have a good start, it is actually slow.
What could be the problem here?

or what do I have to change to improve it?
Thanks
Carlos