65 Barracuda - Idle and no start issues

This could be a LOT of things. Fuel, electrical. XX years old wiring...........

I think I'd start by downloading the closest thing to a shop manual you an find, MyMopar

The barracuda stuff may be separate I forget, but the wiring is quite similar

Maybe "rig" a voltmeter so you can monitor what is voltage / charging doing?

Then rig a simple temporary pilot light on the coil + wire so you can see if the ignition is losing power

Have a look in the dist for condition of points, general condition. Points in good shape? Oil /grease/ rust in distributor?

Could also be bad coil, they typically quit when hot, but often start missing beforehand

How about rust in fuel tank, intermittent loss of fuel? You may have to rig a temporary fuel pressure gauge so you can see what that end of things is doing.

This old, could be anything........junk in tank, plugged/ gummy filter sock. Junk/ rust in fuel line. Rotten rubber tube connectors---one at tank, other at pump up front. Could be pump itself, and could something wrong in carb.