I've long been planning the same conversion using the old AMT Model T delivery truck, so I'll share what I've figured out so far.
Apparently a good many ambulance conversions were actually done on the standard T chassis, since existing inventory was in plentiful supply. The T chassis had a 100-inch wheelbase. When Ford introduced a production truck chassis, the Ford TT in 1917, it was mostly similar to the original T, but with a 1-ton rated strenghtened chassis and rear axle which had a 125-inch wheelbase. There were also optional gearbox choices available which gave better performance on hills and grades.
Google (or whatever) "Ford TT" and "ambulance" and you should be pointed in the right direction, but, basically, I think you've got a lot of latitude. It may depend as much as anything on what timeframe you're going for.
Hope it helps.