The straddle-mounted pinion is a neat design. The 10.5" AAM also has the straddle-mount. Lil' pinion bearing of my K2500 10.5" visible in this photo from my Truetrac installation.
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You're correct, the 9.5" AAM axle has an overhung pinion (no third bearing). But it works well enough. It's not like that two-bearing design hasn't been thoroughly tested.
The lower-mounted pinion may be one reason that the Nine handles shock-loads well, but doesn't seem to handle regular high-mileage all that great. Lower-mounted pinion also increases gear lube temperature, and has more tendency to push the differential case out the back--added stress on the differential side-bearings and bearing caps.
As for gear contact area being greater on the lower-mounted pinion, this is true if everything else is equal. But everything may or may not be equal. Wider pinion/wider ring gear would also increase contact area. I don't know how the Nine Inch ring and pinion gear width compare to the 9.5" semi-float. I haven't had a Nine Inch differential apart since I was in Trade School, other than to replace pinion seals, axle bearings, and gear lube. I've never done "real" differential work on one.
Jim Allen, and Randy Lyman (of Randy's Ring And Pinion) (RIP) published a book--Differentials Identification, Restoration and Repair, ISBN 978-1-4243-2661-7--detailing general service procedures, and the history and various specs for a heap of different differentials.
www.themotorbookstore.com/differentials-identification-restoration-repair-manual.html
Differential repair tips, tricks and performance mods. All 1960-on American axles. By Jim Allen and Randy Lyman. Ring & Pinion Services, 26617. Free Shipping.
www.themotorbookstore.com
They've got a "strength" rating for each axle design, based--I guess--on how big of a tire the axle should be able to cope with. Larger-diameter tire = more strength. This seems to me to be more 4WD, off-road strength than drag race, but it's all I've got. The Ford Nine in 33-spline variety is "rated" for a 33" tire. The Nine in the stronger 35-spline version is "rated" for a 35" tire.
By comparison, a Mopar 8 3/4 is "rated" for 35" tires, the 9.5" AAM is "rated" for a 37" tire. A Dana 60 is rated for 38" tires.
Maybe that doesn't mean much for drag-race use, but it seems appropriate for axles crammed under normal-use pickup trucks.
The biggest advantage of the Nine Inch is the enormous aftermarket support.