I think a 70 series spicer is to big for a 1 ton.
I would look for a 5831 or a 60 series is not much bigger.
I am not sure about using one backwards, I'll have to think about that. As I remember they have straight cut gears, so what's to hurt it?
They run backwards part of the time even when put in normaly..

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-