Ignition breaking up at higher rpm

I put together this little scamp a couple years ago. It has ran great and never disappointed. Now, I'm trying to sell it. Guy shows up, we go for a test drive. It falls on its face. It will idle, won't pull, makes an eerie noise through the exhaust. Not only was this humiliating, it's testing my patience.
It is a 72 Plymouth scamp. It has a 5.9 magnum converted to Carb . I have a Carter fuel pump (electric) mounted low like it's supposed to. Carter 750 AFB . Fuel is filtered at tank sump and after pump.Electronic ignition still using ignition 1&2 .
I thought for sure I had it right out of the gate. Found my ecu loose. Swapped it, polished the mounting and added ground washers. No change. Checked the ballast, loose terminal, swapped w new, no change. Confirm voltage at coil. Good. Check coil resistance. When hot, obtained continuity from primary to secondary winding. Swap with new. Attempt to check gap at pick-up found plate flopping around. Got a reman and dropped it in. Now remember, this thing ran great before whatever happened, happened. Now I'm thinking I was off the mark. I checked the fuel system. Pulled the tank, cleaned it thoroughly. Blew out all the lines. Checked and cleaned pre-screen. Pulled top off Carb, cleaned jets, checked floats , pulled needles and checked springs for bind. It'll start, it'll run it just turns terrible at around 2500 I think. No tach.
Everyone, I know this was kinda rambling but I know you all want what's up and how it's equipped.
I appreciate any help you all might offer. And I won't start another thread on this.
Thanks.