Zack, my first question is, are you gonna get enough participation to make it worth while?
Second, get rid of the "internal modifications" rule. You and I know that you can't police that.
No matter how you set the rules, somebody just gets in under the wire and loves it, and somebody is just over the wire and hates it. That being said, I think 3mph might be too much for pure stock.
JMHO, if I were making the rules, I would go with stock RPM, clevis on factory drawbar at 14", and 2.6 or 2.7 MPH.
That might give a truly "stock" tractor a fighting chance.