1938 Truck Cable to Hydraulic brakes

[38FordTruck]

Rebuilding my 3ton '38 and can get hold of '39 hydraulics. Using both front spindles & hubs from '39 can swap to front axle )38 cable brakes are larger diameter hub/spindle backing).
The '38 rear axle backing plate size to axle is 7" can anyone advise on the '39 hydraulic hub / backing being 7" and will fit and with same axle??

Issue is I am in Tasmania (smaller island south of Australia mainland) - parts are 1895 Kms / 1177 miles away on a station on the "mainland".

Any foreseen issues if the sizes fit.
Currently have everything stripped ready for the "big" rebuild.