moot
looking at the chev cutaway chassis the 139 is the srw. both the 159 and 177 are drw. you could alwys take 2 tires off and use them as spares.:D
the 139 can be had with gvrw of 9600/12,300 lbs. the 159 is 10k /12,300. max payload for the 139 is 5072 and 5152 for the 159. brochure does...