Most of the buses with the 6BT/Allison 4 speeds were 1:1 final drive ratio, unless you get lucky and find a really recent one that has OD. I wouldn't buy into the Allison is tougher than ??? whatever brand you insert either, they do burn up too. There is a reason Chrysler used their own transmission in the Dodge trucks, same thing with the Getrag that often gets a bad rap, fact is NV4500 is not as tough but is definately more popular of a swap when rebuilds are considered. Your best bet is to buy a donor vehicle that has an OD trans. and your going to want to keep the rpm around 2k when considering axle gearing. I know a fellow who has done a bunch of conversions like this one. Very good conversion for you, nearly a direct bolt in with the right donor.