Have you thought about dual brake calipers on the rear end? If not maybe you should, they really make a difference My S/P car has dual Wilwood 6 piston calipers on 11-inch disc and stops really well at 150+ MPH weighing 2850 Lbs. It has 40 spline Mark Williams axle with a Moser steel 4 series spool, I couldn't find a 3 series 40 spline spool so I could run 4.10 or 4.30 gears I ended up using a set of stock Dana 4:56 ring and pinion out of a junkyard pickup Dana 60 I bought years ago to narrow down. Lots of passes on them and they still look good
What size splines on the spool and axles?
As far as the spacing specs on those axles contact Strange Engineering and ask them.