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

if you are making an exact copy of what you have, I would .
1 place rear on flat surface on jacks
2 remove wheel and drums
3 drop plumb bob lines off straight edge against axle flange.
4 mark points, on floor, for and aft of axle flange to get flange line.
5 do each side.
6 drop plumb bob off center of companion flange (ujoint) or pinion nut to floor.
7 record pinion centerline dimension for left and right axles from pinion to each axle flange line.
8 give to fabricator.
If he cant figure out what you need find someone else.
9 install new axle on leafs in position desired, tack and weld perches at proper pinion angle.

get a pinion centerline tool from someone that does rear axle work or buy one from one of the companies who makes them.