The easy way is to clamp a piece of angle iron to the frame flange and let the drill bit for the hole saw penetrate the angle iron wherever it gives the arc in the frame the correct size and position. Then use a piece of black water pipe (NOT GALVANIZED!) to replace the piece of frame flange you removed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!