Pre 67 bus transmisions came with gear set at end of axles called reduction boxes to help with the extra load and weight issues assciated with vans . Buses were cut up and trannys used in bajas because of lower gearing .Most lowered buses go beetle tranny and get rid of reduction trannys . They can be expensive to rebuild and top speeds are modified Ther are 2 types small nut and big nut 66-67 for big nut I beleive the big nut with what is called a freeway flyer gear is good for 65 -70 wich is good enough for a bus but earlier ones limit out at 55-60 max Hope this helps I 'm also not an expert but these facts are pretty close
Geoff or others can add more info if I missed any
