Good question... Interested to learn the answer as well...
Since I can't get my rusted POS 14b semi. axle apart I can't even use the yoke on it to replace the one on my tow rig (14b semi). The 'nubs' were broken off and the caps have to be tacked on.. I wanted to correct that when i did the gear swap..
I'd be a little surprised if it was as simple as FF vs. SF to determine a yoke size. The 14b has been around for so long, and been used in so many wildly different applications, that there is almost no way they would have opted to put the same yoke on all FF's or all SF's throughout the decades. I think you'll have to measure them. Here's a link over to Pirate, toward the bottom of that page they have a chart with the dimensions of the various U-joints.
Sorry I guess we didn't say it but were both assuming 1350 u-joints.. The point was about the yokes (spline count) and swapping from the 9.5" Semi float to the 10.5" FF