How many lugs on your 3/4-ton? The later 3/4-ton trucks have 8-lug hubs and there do not seem to be any conversion kits.

I believe some of the pre-Advance Design 3/4-ton trucks used 6-lug hubs, and if that is correct you might investigate using a disk brake kit for an Advance Design 1/2-ton on the '38. May not work, but might be worth checking out...

Another possibility to investigate is swapping a 1/2-ton axle under the truck to support the commonly available disk kits.

Read up on brake system hydraulics before swapping to a master with different bore. The relationship between master and wheel cylinder bores has a lot to do with pedal pressure and braking power. You'll want to make sure you have those issues under control.

I don't think I'd put disks on the rear with drums on the front. But if properly balanced I don't know that it would not work. But your brakes are too important to take chances.

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...