If you're talking truck 283s, according to the manual, looks like compression ratio (8.5 on LD, 8.0 on HD), camshaft timing, and the oil pan (5 vs 6 quart capacity). It should be noted, there were no 283s in '55-56, and in '57 it was only available in 2 ton and larger in trucks, US pickups and 1.5 tons didn't get them until '58.
