The difference in first gear ratios really isn't an issue. They're both so low it doesn't really matter. You may be a 1/2 MPH faster with the SM465. You'll probably find yourself taking off in what is 2nd more, unless you're on a steep hill.
Both trannies are tough as nails. Hauling/towing not an issue. The 465 is probably the better of the two for being heavy duty, plus it has better internals and shifting manners.
With the 4.57 rearend, you'll be turning about 3000 RPM at 60 MPH, which is about all that you'll want to do in a drum-brake straight axle truck.
BTW- The SM465 has a different front bearing retainer than the SM420. If you want to use the factory mid-mount, you'll need a 67-72 truck bellhousing with side mounts.
Last edited by Whitedog; 05/01/2017 2:49 PM.
