I am not sure which direction you mean by back. The below is a constant for any given scale length.
Bridge<-----------------------------------------------scale length-------------------------------------------------->Nut
Trying to mount something into a neck pocket where the heel is designed to end at the green line with a neck that is designed to end at the red line, the whole lot needs to shift to maintain the scale length.
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Bridge<-----------------------------------------------scale length-------------------------------------------------->Nut
Sketches above are not to scale
Better to make a body than trying to adapt one. Either way, better to have your design worked out before starting.