The 77 and 68 should both have 4.1 ratios and are interchangable....the flange bolt pattern for the 68 will be different than the 77, however, the pinion spline count is the same and you can simply swap flanges.
Don't forget to remove the residual valve from the brake master cylinder.....actually, you might have the single-res setup on a 68, so you outta be considering a newer master anyways....
The 68 uses a ball-socket setup that is useless. The 77 uses the smallest of the fine-spline birfields. You can upgrade these slightly to minitruck/60 series birfs, but you'll need to grind inside the knuckle to get them to fit.
One thing I forgot....your side gears in the diffs are coarse spline on the 68 and fine spline on the 77....something to remember if you order a locker. The side gears from the 77 will work in the 68 center section however, only the shaft spline count is different.
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