Need help with my new 72 long bed

[jrdz2120]

I just got a 72 the other day. I ran it about a year and a half ago in my 2000 silverado 5.3 and was neck and neck. My problem is I picked it up cheap running bad. From what I can tell it has a 350, long tube headers, edelbrock performer intake and 750 carb, hei and 700r4. It idles smooth and revs smooth in park, but when in gear it struggles and misses. So far I have installed fresh plugs, wires ,cap, rotor,coil, and fuel filter. What I have found wrong is header collector leak, 12v wire from battery to alt. is melted and ignition switch if jiggled cuts out sometimes and not sure if carb is bad or was swapped out. any help or input will be great. Thanks.