Mike,
The 8 1/4" axle was phased into production in the early 70's.
It was used in 1973-1976 A-body heavy duty and high-performance models as a replacement for the 8 3/4".
So....If it came out of a 73-76 A-body,it will fit.
No such thing as a "Late 60's Dart 8 1/4 rear".
A-body 63-76 spring perch to spring perch 43.02"
If you were to go to a 8 1/4" you would need a shorter drive shaft. It also will be big bolt pattern.
Since your not pumping out big HP,I would use your 7 1/4", untill it proves itself wrong.