A lot of good information here. Don't know if this will help any but I will throw it out there.
My truck has a M-21 Muncie and a bench seat. I modified a Hurst Indy Shifter to clear the bench seat.
http://good-times.webshots.com/photo/2625896630103882008DsyGRLOn my truck, just as LGriffin stated, the shifter comes through the floor right about at the leading edge of the front bench seat.
The shifter on my truck has a very worn out mechanism. I replaced all the bushings in the shifter arms but it is still very worn. I checked with Hurst and they wanted almost as much to rebuild it as a new shifter costs (it is "factory" Hurst shifter from '65 (bayonet mounted stick). I therefore have bought a Hurst P/N 391 3780 Competition Plus Shifter which is made for a '57 Chevy full size car, haven't installed it yet though. Note that the handle on this shifter (39103780) isn't removable, it's welded. Hurst says this is because of the loading that results from the drastic c-shape that is required.
Here are the spec's on the new shifter:
http://www.jegs.com/i/Hurst/530/391-3780/10002/-1#John
