If your using a spool you need 1" of spool engagement with the axle per Moser, we were able to cut down the splines when I narrowed by rear near 2" per side with my mini-tub. I do have 9" bearing ends and Wilwood drag brakes however.
I have a fixure that allows cutting down the rear and welding in the 9" ends, without this you can not do it properly. The axles have to be cut and the edges filed on every spline. It is best to contact you axle brand manufactor and talk to them before doing anything.
My 325/50/15 BFG drag radial tuck into my mini tub nicely with the modification, I assume you are mini tubbing as well? Why do you want to run a "W" series tire, there are guys going 7's in the quarter on a non w tire....