I have a 63 Polara that I'm installing 3400lb Super Stock springs in. Which rear shocks should I use?

Buy as much shock as you can afford.

That said, we started with the Calvert 9 position single adjustable on our '63 330. They are fine for a car well into the 10's.

We've been 1.36 60 ft. with them.

I bought Viking Crusaders a while back but just haven't gotten the chance to use them much.

As an aside, the modern made in Mexico SS springs do NOT work well for very long. Granted, I ran ours pretty hard in the first year, but they started losing arch and were done by the end of the season! This is in a drag race application. Part #'s 456/457 which is what I believe you have.

This is the handling forum. If a drag application you might find more informative responses it seems in the Race forum.
If indeed a handling question, use a 22" front segment leaf if possible, and upgrade to a fiberglass leaf in whatever spring rate you chose, and save 50?lbs, use a double adjustable shock, like a QA1.
Anything but a KYB.

Depends on what you are using the car for. For drag racing buy a really high quality drag race shock. For street driving buy the best shock you can afford. I use Koni shocks on my '65 Coronet with SS springs.

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.