what type exhaust to use on a 65 cuda commando?

Welcome to FABO!

I guess it depends on what you are going for. If you want original, single, with the wonderful sounding, high buck resonator would be in order.

Back in the day, I put duals on mine, and I put duals on the recent 65 resto-mod I did for my Dad.

Of course, TTI is the best stuff, that is what I have on my 68.