And it can be done much cheaper than a Strange S60, which is based on the Dana 60. You can find one under any 3/4 ton Chevy pickup, and I'm gonna offer the torque arm bracket.

its not a wider rear?

and what are you planning for a torque arm bracket?

I do fine metal fab. And yes, you have to change the housing ends, and get some axles made. It's more money than a Ford 8.8 by the time you're done, but less money than a new 12-bolt.

My 91 454SS truck has the smaller 9.5" 14 bolt diff. It's just as strong. It has 33 spline axles and uses the large GM 5x5 bolt pattern rims. There's also more aftermarket components available such as Powertrax lockers, gears, etc.

I'd say that he's recommending the 10.5 because it has a bolt in pinion housing like the 9 inch Ford. It will be easier to make a torque arm bracket that will bolt on with the 6 bolts that hold the pinion housing in place. The 10.5 is the only rear end that I know of that has a bolt in pinion houisng and loads from the back side.

While a truck is sitting on the ground, how could I identify a 10.5" rear?
How much wider is it than the stock rear?
How good is aftermarket parts availability for it?

Just look at the end of the axle. All of the 10.5 rear ends have full floating axles, which means that the axles can be removed without taking the wheels off. There are 6 or 8 bolts that hold the axles in. They bolt directly to the hub. The 9.5 rear end uses a semi floating axle like the Chevy car rear ends. The wheels and brakes must be removed before the axle can be removed. The 10.5v also has the bolt in ponion housing with 6 bolts holding it in. The Dana 60 does not have this.

Aftermarket parts are everywhere for these rearr ends because they are popular with 4X4 trucks. Yu can get Detroit Lockers and several other differentials for them and many gear ratios. These are heavy rear ends, so if weight is a concern then you might want to look at something else. They will also need to be converted to semi floating axles by using after market housing ends and axles. This will require a housing fixture, good welding skils and some accurate measurements.

Why not get a 9 inch for it ?

My guess is this is being done for $ reasons. Besides, many people are like me and don't want the extra hp losses of pulling a 9" over a 12-bolt or S60. I don't even run a 9" in my 10.5 outlaw car.

They are way too heavy for me. I've narrowed a few for early flatbed trucks, but i would not want one in my car...3-1/2" dia tubes that are up to .492 thick.
Same hypoid offset as an 8.8 though, much more effecient than the 9"s 2.38"

yes, it's a heavy rear end, no doubt. I considered the 9.5 from the 454SS pickup, but those are very rare. Around here, they can't be had at any price. The 30 spline axles don't seem very strong, but they're larger splines than the 30-spline 8.625" 10-bolt and 8.875" 12-bolt. There's a guy here with a modified Duramaxputting down just over 1000 ft-lbs at the tires, through a stock 10.5. It survives. this 10.5 isn't for any tire less than a 325/50R15 drag radial, and would be good with anything up to a 33x21.50-15 Mickey Thompson Sportsman Pro. But I'm going for a twin turbo 511ci BBC in my '82 Camaro, and I love those first 60 feet. Also, as has been mentioned, doing a torque arm bracket to the pinion support is a big advantage over the Ford 8.8, which is now going to my '84 Trans Am.