I have both 8 and 7 tooth on hand. I suggest the 8.
Unfortunately thre is no better 7 tooth shaft than the regular one. I tried and tried and tried and tried and tried to find one. Alan searched harder than I did I think. From 94 through 98 all of the 7 tooth 4R70W shafts are the same. They didn't make a 7 tooth in anything after 98 as far as I know.
The 8 tooth is a better shaft all the way around.
01 F150 5.4L 4x4 and a 98 Mustang GT vert sitting engineless and without transmission or converter