Some rear ends are easier to narrow than others. Most c clip axles can not be resplined. The 9 inch Ford, Dana 60 and 8 3/4 Mopar axles can usually be resplined. I would be careful about using resplined axles. They are not as strong as axles that are made to the correct length. If you break an axle on that trike you are going to be in for a wild ride that you might not survive.
You do need a housing fixture to align the housing ends with the differential bearing bore. If you don't get this straight you can have all kinds of problems.