Joe,
Indeed I do have a Sadev gearbox in my Superlight SV. As Tom says, I added a Geartronics Paddle Shift kit to the Sadev. Just double the price of the Sadev (ouch) and you will have one fantastic system. Mog-Racing sells a Geartronics setup with all the parts you need to connect the Sadev.
With the paddle shifter, the shift lever is eliminated so there is no way you would need to modify your chassis. I don't recall just how far the Sadev gear lever moved, but I don't think it was all that much. I agree with Mike, get it and see.
The Sadev has a weight advantage over the Quaife too!
If you plan on a lot of street driving, I'd stay with the 6spd. But for the track, WOW is it fun. The Sadev with the flatshift setup worked well for upshifts, but the paddle shift kit is a whole different world.
If you want to make the trip to NJ, you are welcome to drive my car before pulling the trigger. Warm weather is just around the corner.