They came new with both 1300 and 1600 single-carb engines, so unless you keep your foot to the floor all the time, I wouldn't worry too much about increased torque. I had an F12 with Perkins diesel, which has 75-80 ft/lbs of torque, drove it flat-out cross country.
The u-joints are huge, and I would think are up to the torque if in good condition. Much bigger than 105/115 type, and not easily replicated in the US. I had one cup that was slightly loose, and the driveshaft shop that examined it for me was pretty awestruck about the size and strength. I had the whole LF hub and driveshaft off, and it's very substantial stuff. Again, if the condition is good, I wouldn't worry about the extra torque unless you build a race motor and run it wide open.
See my F12 thread for pics of the hub and u-joints.