It may be that the power valve in the carb is stuck. The power valve enriches the mixture under low vacuum to give the engine some power and then leans out once the vacuum comes up. I had a Willys Jeep with a Buick 225V6 with a 2GC and I found a bent power valve piston hanging up in the bore that intermittantly caused crummy acceleration and terrible gas milage, because it would stick closed for a while, break loose, and then stick open for a while.
Also, check the total mechanical advance of the distributor at 2500 rpm, you may not be getting enough ignition advance to make any power. The weights may be stuck or rusty and hanging up. If the distributor sat for a while it could happen.
