I have an old 1 ton Dodge pick up rear axle. I disassembled it & cut off the old full floater ends. I made up my own rear axle narrowing fixture out of some steel discs I made up for where the center section bolted & made up some steel housings end flanges the slip into & bolt to the after market Strange housing ends. All four discs have a 1-1/2" bore that I slide a 1-1/2 solid bar thru the axle from one end to the other to hold all suare & true.
The car is mini tubbed & I have Caltracs w/ Split mono leafs w/ a Doctor Diff 1" relocation kit. The wheels I have are 15" x 12" Welds with 315/65/15 Drag Radials. Just mocking the wheels in the wheel tubs everything fits nicely. Its tight but it fits. Mocking it up with a wheel flange to wheel flange I have a dimension of 54".
My questions are. How do you back figure the o.a.l. length to cut down the axle housing? Then adding the width of the axle flanges.I plan on running Strange axles & rear brake. Then I will need to add the length of all of this.
So what's the best way to get it all correct the first time?

The axle flange standout is 2.5" from the housing flange so subtract 5" from the overall axle flange-to-flange width to get the housing width. Add the drum or disc thickness to each side for a precise overall width dimension.