Engine Cuts Out

[Sloan66]

Lots of questions, just got truck running. Truck is 71 w/ 307 and 2bbl trauck sat for 15 yrs before I got it, prior to start up I replaced all fluids, rebuilt carb, new timing chain, all new hoses and gaskets, installed HEI with new wires. Fuel lines and tank flushed, inline filter placed bewteen tank and pump.

Truck start and runs fine, drives good, but randomly in third gear the engine cuts out, any ideas on cause?

Plugs are gapped .045 and timing is a 8 BTC and I did remove the resistor wire to distributor and replaced it with 12 ga. so it plugs into to factory location where the resistor wire did.