A quick measure would be to measure the distance from front axle to back axle, multiply this by 2, and then add the circumference of 1 tire. This will give you the bare minimum length of the track needed. Of course, if these are steel tracks, you will really need tracks longer then this number since there should be a little sag at the top in order to not burn out your bearings.Any quick help would be great. I'm looking at a set that came off of a 751. But I'm pretty sure they won't fit. Well I'm guessing