I'm the man to talk to about the 26 spline input shafts.
They can handle a good amount of power, however me and a couple other members bent a few.
It isn't that hard to install. "Dana" has a right up on it over on Mfords.com.
You need a dial indactor to check input shaft end play. I've done it twice now, I had the trans out/changed input shafts/trans back in, under 4 hours or so...
The hardest thing about it all is the top starter bolt (mine now in the trash) along with the 2 top trans bolts.
FYI, u need a 26 spline clutch, im sure u knew that already
Kevin