There are kits on the market that will put either 6-lug or 5-lug disk brakes on your axle. The front axle is the same from '41 through '55 1st, though the spindles vary a little and the drum brakes do change from Huck to Bendix in the 50's.

First point is that you don't need to figure out your own approach to brakes a solution can be bought from most the old truck vendors.

Second point is that it may be easier to find a replacement axle than spend a lot of time and money on your existing axle if it has problems.

If your 350 is relatively mild I would not mess with boxing the frame unless you've done other mods that weaken it. You may find you need to remove the crossmember that the stock bellhousing mounts on in order to fit the transmission under the floor.

That crossmember also provides support for the spring pivots and if you remove it I would then look at ways to add that support back in.

Those are my first thoughts.

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...