Yes I would think you'd have to use custom axles. The other issue is that's getting your spring perches out there to the tapered part of the tube so you'll have that to deal with as far as wheel back set, U bolts, etc.
Well I figured that would be solved with running the spring relocators inboard kit - which I was already planning on doing because I also wanted to Mini-Tub that car too and it looks like I might be doing it all sooner than expected if I want to run/install this modified shorter rear.
I'll still wait to hear what your inside flange to flange measurement is before I decide if this is the way that I want to go. I also have to go price out how much its gonna cost to get custom axles made as well.