HELP! 66 cuda idles fine, but not when in gear

And if you want to get best use of the V-can;
rotate the Distributor exactly two towers CW, and then rewire the cap Exactly two wires CCW. Then reset your power-timing. Now the Vcan has put the D into a position for easy timing changes.
It's not real important but if you open your hood at a car show,like it is, your newbe status will on display.

After you plumb the Vcan to the Sparkport, you will need to reset the timing and reset the idle-speed.

Ima guessing the other valve cover has a 2-way breather on it?

The hose on the PCV should be specifically for that. If yours is fuel-line, the engine will suck that flat while cruising.