1965 Barracuda MC conversion to Dual Resevior - Push Rod Question

You need to use a disc/drum MC like in your picture. Sounds like you have a bunch of brain deads at Oreillys. Do not use a drum/drum MC. Use the right part.
www.rockauto.com is a good source for parts. Don't be so concerned about the size of the bore. The only real difference is the amount of pressure necessary,on the brake pedal.
You will be fine with whichever one you get. Look for a Disc MC out of a 73 Dart.

Your pushrod will work fine. You need to take the piece off the end of it. Use a vice.
I take the pushrod out, before removing the MC, by pulling back on the brake pedal.
Try to save the rubber grommet that is on the end of the pushrod. If you can't,all rebuilt or new MC's should come with one.

To install the MC:
Put rubber grommet into MC...Hint,use a similar size socket to push it in.
Install MC to firewall.
Bolt pushrod to brake pedal.
Align pushrod into MC,and push.
Readjust brake switch
All done.