Dennis,
I just finished installing the rear axle this afternoon (less than an hour once everything was set).
I did find somewhere else on the forum that the standard measurement for the outside bracket spacing is 61-3/8" so I think you're measurement is good. I had 80-1/8" on my outside hub face which should be very close to your measurements as well. My point is, you're well within the ballpark.
I just posted pictures of the second axle install (didn't take any of the first) and I think you'll find them enlightening (as well as the narratives). All is very fresh at the moment.
Since you're working with slots on the axle brackets rather than holes, I think you want to use the mounting plate as a measuring rod to be sure that all four axle points are even (same place on frame). I will be double checking these when it dries out tomorrow and may need to adjust. (The price of not using OEM)
We'll be in touch...
P.S. - I just noticed your original axle has mounting holes on each side. Did it use 4 bolts per side? (8 per axle)