Won't run at mid-range to high speed

1981 Ram 50. 2.6 5 spd.
New "reman head", timing set, water pump, ignition module, plugs, wires, cap & rotor. Ran great for a few months, then about 3 weeks ago it
started running poorly, woudn't rev past 2500 and if you tried to accelerate, she just stumbles and won't go. Let up on the pedal, she'll slowly build speed as long as it's not uphill.
Bought a kit for the Mikuni and cleaned it and put her back together. The kit didn't have a couple of diaphrams and they didn't look too good but we put it back together and it still ran like doodoo.
So, we put a brand new weber carb on it. EXACTLY the same.
Any ideas guys? Hate to junk the truck with close to 1000 dollars in it in the last 8 months.
Thanks.

Did you replace the fuel filter located right outside the tank? Can be reached from driver's side by reaching under bed.

Sure did Josh. Thanks for the reply.
FYI, it got so bad that it won't rev at all even sitting still. This was before the carb cleaning and the weber has not helped, guess I'll have to open the exhaust pipe and see if that is where the gremlin lives.

oh yea I forgot the older trucks had a catalytic converter inside the exhaust manifold and they clog and go bad, especially when the truck is as old as it is. I remember someone having the same problem not too long ago and that was it, just smash it out if it is and you should be good.

Gremlin was in the manifold. Bashed him to bits and poured him out. Runs 1000% better.
(The above poster did not destroy or modify his catalytic converter in any fashion that would be considered a federal crime. Simply removed an obstruction from the exhaust system.)
Thanks for the info.