And if you're OK with setting up used gears, 4.10/4.30/4.56s are available everywhere on the secondhand market from mustang guys trading up or down ratios.
Easy to shorten left side 3" and install a RH axle on the left side. Honestly if i wasnt so close to having everything to assemble my 8&3/4 for the 67 notch, i would have done a pick n pull exploder rear, and never looked back.
We will be doing that one on the 69 notch, and using dippy fmj spindle front brakes. No bump steer if using geometry corrected tubular UCAs. Hey Txstang84 ask Jim Scott about that one lol. He gave me a tutorial on mopar disc brake spindles one day. FMJs are extremely close to 73 up A body spindles, exact actually except for the upper ball joint attach point being 1/4" taller.