One of my friends used the Downey adapter behind a GM Vortec Crate engine (330 something horsies and somewhere around 400 LBS torque) in his '73 FJ40 with downey 4" springs, and 35 inch tires. Snapped the downey conversion shaft that was installed in the tranny (broke clean in two) This happened on the street with his foot in the skinny pedal pretty good, and just the slightest little bump in the road to give us a little bounce... BAM!! and then no more gears. had to walk back to my house and tow his 40 home... This was only a month or two after getting the thing running, and Downey did not honor their warauntee, and my buddy had to buy another $$ downey shaft and have the whole tranny torn down again... Really disapointed in Downey over that one. Got a close look at the shaft again not too long ago, and it snapped right at a big stress riser cut into the shaft right behind one of the splined ends- looks like a groove cut for a snap ring or something. If going the Downey route, make sure you know what you are getting into... see if they have redesigned the shaft, and how they will treat you when you come back with broken parts.
My friend's rear shaft was pretty short, but everything seemed to function when it wasn't breaking...