I am missing information or something here apparently.
The receptacle is for the welder, is it not?
If the receptacle is rated at 50 amps, you cannot supply it with 65 amps.
The maximum OCPD can only be 50 amps on the receptacle.
There is no need to up to 60 amps on the circuit unless you think the 50 is inadequate.
I think you are wanting to size the 50 amp based as continuous duty as in other areas of the code.
But, the code specifically states:
Even under high-production conditions, the loads on transformer arc welders are considered intermittent.
So, that doesnt come into play with welders, only the 200 percent maximum previously posted is applied