I can give an opinion Squid, seems here a few don't like what I have to say though.
If you want to make sure you have a class to pull in can't go wrong with an altered 12,14,16 kohler. You would surley have alot of competition in the altered 16hp kohler class.
On the front tire thing, pick up a set of 6 inch wheel barrow tires, you can put them on a set of two piece cart rims, with a set of cart hubs with 3/4 bearings in them. Matter of fact I bought a set of the rims from you awhile back.
As far as rear tire you would do well with a set of carlisle super lugs or the firestones if you can find a set. Single cut or double cut. Just depends on the class and if it warrents the need for the pulling tires. I think the carlise's are 4 ply. Just remember if you use a lug tire to make the rear fenders adjustable because a true pulling tire is larger in hieght then a lug tire.
This is a carlise double cut super lug with a 6 in wheel barrow tire on kart rim 2 inch wide halves.