I installed a welding circuit with a 60a breaker in the lower part of a split buss residential panel and put a 70a breaker (max size according to the panel specs) feeding the lower part of a the split buss.
The the welder draws 92 amps max. I didn't want to go over the 70a split buss feeder breaker, so I told the customer he probably couldn't run his welder at maximum without possibly tripping the breaker. Everything is installed up to code, but is this a good idea even being legal?

(This is the same panel I mentioned in the General discussion area about the ("no main disconnect".)

A lot depends on the duty cycle of the welder. You need to or at least should look at Article 630 which specifically addresses welders.

Welder outlets, the conductors that feed them and the overcurrent protection for them do not follow the 'normal' rules.

I'll check that.
Its a Miller Tig welder

93Amps would be extremely heavy current for a MIG welder used at a residence. That is most likely an instantaneous value.

If it is a Miller MIG running at 120Volts then 60A is way overkill. Just what kind of plug does it ship with?

Dig into their spec sheet; visit their web page.

Welders get their own section in the NEC. It's worth looking up.

The nameplate will list a "duty cycle." This bit of info in turd is used to select your breaker and wire size. In theory, one could run #12, on a 100 amp breaker, if the duty cycle was low enough.
DOing so would probably be foolish, though. Replace the welder with another machine, and you're likely to have to re-do the entire circuit.

Have you considered putting the welder breaker in the upper portion of the split buss panel and moving some smaller load down like maybe the water heater or dryer(double 30)to the lower portion?

Thats a great idea. Sometimes the obvious just flys right by ya. I check the loads on top.
Thanks,