Welcome!
We offer a sterling 10.5 conversion, but most late model cutaway chassis have a dana 70 full float axle which has plenty of weight rating. Sterlings after 2006 are the only ones wide enough to work under the vans. 06-07 sterling is the best because it is much easier to make the E brake work properly. They are also the same width as the front axle we use.
Actually every van we build is leaf spring in the front. There is a TON of reading you can do on which is best on this site and others, but the simple answer is that the coil spring conversions just do not work that well under these vans. They are plagued with various steering and handling issues. Each set of leaf springs is built per vehicle, so they ride very very good. Turning radius is pretty much a non issue. The amount lost is almost not noticable.
Let me know if you have any other questions!