I made Scotts rear engine rider with big tires for back and small in front all it did was wheelies. It was also very sketchy going around the corners cause it keeped leaning and stuff. It had 10.5 brigs flathead in it. With i think 8inch on engine and 2inch pully on tranny. It has a mtd junky tranny but some how from all wheelies we did stuff it still runs like a champ. We keeped the pullys in the same spot though and how belt was set up to. Only thing ill be worried about that rear engine racer is my you know what hitting the steering support or me flipping off the back hitting a bump or something. Even though i built mini and it dose wheelies, I feel much safer on it then the rear engine rider even though they look fast when they race like england and stuff.
Just what ever you do dont try driving up the back of the truck on ramps with it. My freind learn then hard way lol it was really funny cause he asked me to help as he got stuck at top of it and slowly falling on him, as i was laughing so hard. It flipt on top of him and he learn why not to do it quick lol.