Front or rear measurements on the sub frame outside to outside? They do vary due to the width of the sub frames at the front and rear not being the same
I'm going to assume that your moving the leaf springs into the rear sub frame, correct? If so you can do that three different ways, I've done all three.
Move the front hanger into the center of the existing frame, move it into the inside of outer frame rail (most clearance gained
) or move it in so the inside edge is in the inner side of the frame rails
If you want to look at my 1969 Dodge Dart GTS the next time your up this way you can
It has them moved in the last way so the springs will clear the stock gas tank with a little
on the passenger side of the tank
My 1966 Coronet has them moved into the inside of the outer frame rail, you can look at it also