welcome to the site! First off, let me say, it is your truck, and ultimately, do what you want with it! However, some will come along and remind you it is only original once (that does not stop me from upgrading though).

I would be inclined to go with the 57 truck 235 and SM420 since it has a pressurized oil system and the SM420 is about as beefy as you would ever need.

The truck is pretty narrow up front and the 6 cyl will give you a bit more clearance on the side for steering etc than a V-8 would.

Mike
1955 Chevy 6400 ex-flatbed (no bed now!) sold September 2023
In the Stovebolt Gallery
1958 Chevy 6400 flatbed W/dump
In the Stovebolt Gallery
1959 Chevy Suburban Owned for almost 20 years, Daily Driver -- sold May 2016
In the Stovebolt Gallery

Sufficient unto the day is the evil thereof