Power loss at mid to high rpm

[psuautotechstudent]

So I bought a 68 chevy with a 250 I6 this summer. I've noticed when I drive under heavy acceleration I get a popping from the exhaust that coincides with power loss. It happens at about 3 or 4 grand (estimate, no tach). I recently cut off the exhaust behind the cab and put a glass pack on. I want to say I could notice it before that, but the exhaust makes it much more noticeable. I have an HEI dizzy on the way and plan on replacing plugs and wires and of course setting the timing. Think that will make a difference? Anything else I should check? Thanks for any Help