The axle bearing size isn't determined by the cubic inch size or horsepower of the engine. It's determined by the GVW of the vehicle.

Small axle bearing backing plates won't bolt up to a large bearing pattern axle housing end flange and vise-versa.

TKOPerformance wrote:

So there's no downside to the small bearings in a light application like a Mustang?

Even 428 SCJ, Boss 429 and the 429 SCJ in the heavier '71 Mustangs all had small axle bearings in the rear ends. Again, it's a function of gross vehicle weight and not the amount of horsepower produced by the engine that the axle bearing size is based on.

It doesn't really matter which way you go --small or large axle bearing, you'll never have enough weighted load on the Mustang that the small axle bearing won't be able to handle it. If this was a truck or a full-sized Ford passenger car, that potential would be different.
 

Around 1967, the Galaxie was a much heavier car than previous Galaxies. Many of these later Galaxies were running a Ford 9-3/8" rear end (very close cousin of the Ford 9-inch but not the same as).  These rear ends utilized large axle bearings.

The 9-3/8" Ford rear ends were also optionally available in the Ford F100 half ton trucks from 1968-1972. This rear end came with two different 31-spline axle versions: a 3300# axle and a 3600# axle. The 3300# axle would have been installed in a 9-3/8" rear end with an open (single track) differential. The large axle bearings on this 3300# 31-spline axle measured 3-9/64 O.D. x 1-17/32 I.D. This was a ball bearing configuration.

The '68-'72 F100 9-3/8" 3600# 31-spline axle would have been installed in a 9-3/8" rear end with a Traction-Lok differential. The  bearings for this axle measured 3-9/64 O.D. x 1-5/8" I.D. This was a roller bearing configuration.

I have a pair of '71 F100 9-3/8" 3300# 31-spline axles in the stock 9-inch rear end housing under my '69 F100 Ranger, that replaced the stock 28-spline axles/single track 9-inch differential. I installed these 31-spline axles to be able to install a factory Daytona 9-inch N-case 3rd member I have that has a 4-pinion, 31-spline, Traction-Lok differential with 3.50 gears.

A 9-inch 3rd member will fit in a 9-3/8" rear end housing but, a 9-3/8" 3rd member won't fit in a 9-inch housing ....well, at least not without notching the flange for ring gear clearance where the 3rd member mates up to the housing.

The 9-3/8" rear end really isn't a very desirable rear end --other than in the case of the truck versions to get the 31-spline axles from to install in a '68-'72 Ford F100 9-inch rear end housing. Gear-wise, ratio choices are very limited with the 9-3/8" and there's no Ford or aftermarket support for new gears to install in them.

A Ford 9-3/8" rear end is easily identifiable by the 3rd member. It will have two widely spaced vertical ribs and a top rib that curves downward on the left side.

This 9-3/8" 3rd member is from a Ford full-sized passenger car, as denoted by the cast-in 'hood' sticking out over the companion flange. The truck 3rd members do not have the 'hood'.



....Daytona N-case 31-spline Traction-Lok 3rd member I installed in the '69 F100 9-inch rear end under my truck with a pair of '71 F100 9-3/8" 31-spline axles.

Back to the 9" bearing size, my experience with a 9" rears, one from a 58 Wagon (big bearing) and a 59 passenger car (small bearing), both were 28-spline axles however, the big bearing axles were much bigger then the small bearing.  Brake backing plates are not interchangeable due to the bearing OD size differences and the backing plate to rear end bolts sizes are different, 3/8" vs. 1/2".
I bought a 9" rear assembly from a 69 428 Cobra Jet Ranchero and it had small bearings.
My Father's 61 4-door Galaxie had the big bearings (292 V8, 3-speed w/OD & Posi).
Not sure any of the above is helpful to you, but if I would go w/the big bearing only because the axles are also bigger and no matter what HP and traction you have, you shouldn't have any breakage issues.
I'll also note that if you go with disc brakes on the rear only the caliper mounting bracket is different, in order to accommodate the different bearing sizes and bolts.