how much trouble to....

B/RB in an A with PS don't play well together. Does the car have headers or manifolds? Might look to Borgeson Box

I switched from the slow as hell MS to a Stage III power box. But I have a 340 and room is tight with headers.

You need:
PS Box (preferable sent to Firm Feel or Steer and Gear)
Saginaw Pump (easier to find)
Pump brackets
Lines
Column for PS or the shaft to swap out yours as they are shorter (depending on the year Flaming River makes a drop in column the drop in B-Body fits, it just needs the D shaft shortened.)
Coupler, MS and PS are different
correct pullys