If you are going to have the bolt pattern changed, same shop can turn the center register down as well.
If it was up to me I would opt for an 8 3/4 from the correct years, breather hole location dictates, that way you don't have to remember that you have a 75 8 1/4 dart rear or a e body 8 3/4 cut down.
But like grragecat mentioned it is not cheap.
You will need (For either swap):
- Housing
- Center section
- Axles
- Backing plates
- Drums
- Spring peaches
- Correct U bolts
- Brake tubes
- Brake hoses
- Correct or modified driveshaft.
- U joints that match the centered section and transmission yokes
If you go with a cut down 8 3/4
- Cost to narrow.
- Spring perches
- Cost to streighten housing
- Cost for axles
- Maybe machining to change to SBP
If you go with 8 1/4
- Cost to narrow.
- Spring perches
- Cost to streighten housing
- Maybe axles
- Maybe machining to change to SBP
Unless you can do most of the work yourself your looking at 1000 to 2000 regardless what you do.
Simplest route is real A body 8 3/4 complete