Upgrade to LBP - Ordering New Axles???

I have a 71 A-body with the original 8 3/4 rear with the oddball 10" x 1 3/4" drums and SBP. I want to upgrade to LBP with Moser axles and the 10" x 2 1/2" drums. I am assuming that the axles will need to be longer since the brakes are wider. How do I specify the dimensions for the new axles. Do I just tell them exactly what set-up I have and they will know what axles I need. I don't see a way to mock-up everything and get accurate measurements. Any advice? Thanks.