Thanks for the info.
I'd like to compare your proposed setup with the rules for Article 630 from my '99 copy of the NEC. The rules in this article are pretty basic so they may not have changed over several code cycles.
Along those lines, what length will you be using for your #6 input wire? Usually you want less than a 5% voltage from load center to device. Here's one formula for figuring this
http://books.google.com/books?id=UvmE_6XsszcC&pg=PA110&lpg=PA110&dq=mullin +K+cma+electrical+residential&source=bl&ots=4eJvlS jwdr&sig=8OELhLvPI41xLDKTl2o4uYYxl10&hl=en&sa=X&ei =D_aCUO_jHIS-0AHGoIHoDw&ved=0CDEQ6AEwAQ#v=onepage&q=mullin%20K% 20cma%20electrical%20residential&f=false
but deciding what welder current to use for this may be tricky.
For ground checking I guess you could check the ohms between your house ground and a sanded-clean spot on the upstream metal pipe of your water meter.
Footnote: Using your 67A @ 35% duty cycle, 150W idle current and the formula from art 630, the current your welder will draw while the arc is struck is 113A.
Using copper #6 AWG, 26,300 CM at a 5% voltage drop from the 230V you can have up to 112' of cable, so says my spreadsheet.
If the NEC permits you to go smaller on the cable due to insulation temp ratings for a 67A RMS current, I'd do it but you'd also need to go shorter on the length to keep the 5% voltage drop guideline.
With this much current the voltage at your load center may drop 3V or so (more if your house is far from the pole transformer) so with incand. bulbs in your house you may see lights blinking.