There are several different bearing sizes in a Ford 9" you will read the terms big bearing or small bearing ends. This is referring to the semi float wheel bearings that press onto the axles shafts and support the vehicle at the ends of the axle housing.
Then there is the the carrier bearings in the third member that is seems like you are questioning. Most oem third members had carrier bearings with 2.892" o.d. and 1.625' i.d. where the bearing presses onto the spool.
The aftermarket developed third members where the carrier bearing o.d. is 3.25" and the i.d. is 1.781" on the spool side.
The bigger bearing third members and bearings allow for a bigger 35 spline spool.
The o.d. of the 35 spline axles shaft is 1.5". The o.d. of a mini spool or 35 spline aftermarket spool (made to fit in a stock third) is 1.625. That is not a lot of material left between the bearing and the shaft. It can be assembled with an stock third and 35 spline spool but will be a lot weaker than moving up to a 3.25" bearing third and big bearing 35 spline spool.
These are the costs of getting a 9" strong. I can be done and works well. It is almost easier to have all the parts delivered to you door and built vs sourcing an axle to modify.
Fab housing, nodular third, pinion bearing retainer, spool, gears, shafts and full float ends are the icing on the cake. BUT costs more than a 14 bolt.
