Ah... rear shocks.

Might need longer shocks with a +2" lift, but you really ought to measure when you get the springs installed. Shocks that are too short aren't as big a problem as shocks that are too long - if they extend all the way, they pick up the axle. If too long, they bottom out. You can get some specs on the OEM shocks and see how they compare to other models or PN's in a shock catalog.

When I was looking years ago, the info I got on the 65-72 C-body shocks was this:
Extended Length = 22.5" (average of a few sources)
Compressed Length = 13.5" (average of a few sources)

FWIW, A-body rear shocks are actually LONGER than C-body rear shocks by about an inch compressed and half an inch extended.