[RichardJ]

You're going to adapt your SM420, that dates from 1947, so that you can put more modern disc brakes on a '67 Eaton axle? Even at that, the calipers are going to be skimpy car calipers on a 3/4 truck.

I don't see it?

My '67 3/4 has disc/drum and a SM365 (original 3spd). If i considered any change, it would be one that proved real improvement to the truck such as a NV4500 5 spd.

The NV4500 was available with a tail housing mounted driveshaft brake. Might be easier to find than a SM 420 brake

I'd be so excited about having the 5spd, I wouldn't want to ruin that thought by thinking about car calipers on the rear. I'd keep the drums and use a regular NV4500.