Dart 8 1/4 to D60 lots oh stuff to figure out

You better measure you 8.25 because I bet it's longer than 51.40. That's from that POS chart. I'll bet it's wrong. The rear from drum to drum is about 57 1/8"

You're best bet is to put the wheels and tires up under the car and measure exactly what length you need.

Did I send you a copy of the axle lengths program? If i did, use it! From those numbers, it looks like you have it.

If you are using standard mopar drum brakes for BBP on the rear, the spreadsheet will give you the desired axle tube lengths from center section insertion point to flange. That will give you the required pinion offset. If you are using a disc brake set up, the hat thickness will change your measurements.

The bar length is the minimum length alignment bar you need to do one side at a time.

IMHO, TOTALLY disregard anything to do with the old rear axle measurements. It will do nothing but confuse you related to the desired goal. The only caveat to that, would be if you actually bolted the wheel/tire combo to the old rear and measured how far in they need to go.

BTW, if you leave the rear spring in the factory location you are limited as to how far you shorten the rear because the U-bolts will start to hit your housing ends. Looks like you have a factory 68-70 B body width 60.125, which shouldn't be an issue.