Is this for pulling within NATPA or USAP guidelines?  Everybody keeps giving different answers, so I cant tell.  If so, I can tell you that I have pulled an 88 gas from the 4000 lb. class up to the 6500 lb. class for probably 15 years, and it has won many times.  It is virtually stock, I am pretty sure that it has an overbore kit in it, but I dont know since we have never had the head off.  This tractor was stuck, and left for dead in a field when we got it.  Poured some penetrating oil in the plug holes, broke it loose with a prybar on the flywheel, poured some gas in it, and started it, and have been pulling it ever since.  What I am getting at is dont let anyone tell you that you have to have some big dollar ride to pull.  It is more about putting what power you have to the ground than it is how much power you make.