Dart 8 1/4 to D60 lots oh stuff to figure out
If you wanted to know what the Dana axle lengths are from the housing center, add 8.75 to each tube length. The 17.5 center section width is a pretty standard number. You may want to measure the center section, if it is different, input that value into the appropriate spreadsheet cell.
You can use the brakes off the 8.25 rear. They are will work fine on the Dana after the car ends are installed. The Dana spreadsheet section is set up for standard BBP standoff distances.
A quick and dirty way to find out your max rear width. Measure the drivers side tube length from center to where the end attaches. If using moser 8400 ends, add 2.25" or measure the ends you have and add that value. Input overall lengths into the spreadsheet until you hit your max drivers side tube. Make sure the pass side has at least enough material as well, usually not an issue.