Not quite that simple.  Who knows what's been done to the electrical in the last 45 years.  Some dumbass may have replaced a 15amp breaker with a 20 because he kept blowing the 15 and is too stupid to realize that the 15 was there to keep him from burning down his house.  And then the OP puts a 20 amp load on it(which is too much even for a 20) and catches the wiring on fire in his wall.

OP, if you're looking at a serious operation, I'd look into spending a couple hundred bucks on an electrician to make sure your wiring and breakers are properly sized.  Maybe you have him install a few dedicated 240v plugs, PSUs wil generally be more efficient on 240 than 120.  Also, never load a circuit more than 80% for continuous operation.  So a 15amp circuit should stay around 12amps.

Get yourself a killawatt, it's cheap and incredibly handy to determine what your usage is for a particular machine.

Don't burn your house down to mine a few coins.....

Good luck!