I recently swapped a Richmond 5 speed into my '66 Special. It was a fairly easy swap, requiring little modification.
This is not the new overdrive 5 speed that was just released, but the Street 5 Speed trans that has been avail for many years. It features a 1:1 5th gear, and a 3.28 first gear. It works great with the stock 2.78 rear axle gears and 300 engine, and would work well with 2.41 or 2.56 gears behind the extra torque of a Nailhead.
The floor did not require any modification, but the shifter hump did need to be massaged to clear the Hurst shifter. The stock crossmember and mount was used. The crossmember did need to be moved back slightly. On my non-boxed Special frame, I was able to use one existing hole, and only had to drill one more hole at the far end. GS and convert frames might need the crossmember mount extended.
I liked the swap so much, I bought another 5 spd trans to install into one of my '66 GS's! I hope to have that one on the road this fall. Should be fun!