Help Needed- Engine Cutout

[Chance66C10]

While driving my 66 today the engine would cutout at 2500 rpm when in third gear. I was able to bring it past 2500 rpms in 2nd and 4th, but for some reason every time I would reach 2500 rpms in 3rd it would cutout. If i let off the gas it would come back on and chrip the tires a little. If I kept the accelerator down and try and pull through it the engine would completely die and I would have to restart the truck. It runs fine as along as I don't reach 2500 rpms in 3rd. Any ideas as to what is causing this?

Truck Info:

1966 C10
307 engine out of a '68
Muncie 465 transmission
Electronic Ignition


Thanks,
Chance