For SS springs - whichever shock you use - they need to be taller than stock.
IIRC, '69-'70 Imperials used the length you'll need, and supposedly D100 fronts also work.
It's a good idea to take a measurement at full droop and at max compression.
You don't want the shocks to limit travel.
Also, in my case, the pinion snubber wasn't needed.