On my 65 I swapped to a dual res master and use the original pushrod. It works fine and the pedal travel is as it should be.

I would check everything else, and make sure it has been bled correctly, including bench bleeding the master. If the adjusters and cables have never been changed the brakes may not be adjusting correctly.

If you need a shorter pushrod for some strange reason, the master could be shimmed out from the firewall with a flat aluminum spacer.

Cutting and welding the pushrod is not a good idea at all and not safe.