With a 4.1:1 axle and 165/82 15" tyres, ie standard 4/4 70s and 80s a four speeder is doing 4000 at 70mph. So beware of your speedo accuracy those who have reported differently!
Not sure of the type numbers, but a Ford gearbox as fitted to the XR4i is both stronger and has ideal ratios for a road car being 1,2,3,4 as per the 2000E box (which matches the engine perfectly) and the addition of a fifth gear. The 2000E box in both three rail and single rail forms were fitted to 4/4s from about 1968 to 1973/4 and are much superior to the later boxes as on these first gear is too low for a sports car.
But the XR4i box is a heavy box, and you will need chassis modifications.
I've driven a normal five speeder box equipped Xflow and it seemed ok to me, but as it was somebody else's car and he was in it at the time, we didn't go terribly fast!
