Low Power Truck; Unfortunatly
The truck you mention is a good work truck but probably a weak tow vehicle as the 5.7 (350) installed has but 210hp and torque to match. Very low. I have a 454 of the same vintage an its original rating was 230hp! A bit more torque but still rather anemic. The tow on flat land would be OK in all liklehood but hills, let alone mountains would be a killer. A 4.10 rear would be better by far and replacing it would be easy with a complete salvage yard donor. Just swap axles. Quicker and cheaper. Make sure you get the correct 14 bolt rear (there are two). There should be a tag on the rear, yours too, that indicates installed ratio.
Minkus: Be careful bigger tires reduce the numeric ratio. I too have forgotten the formula but a search on Google should bring you to a tire mfg site with the info you need.
Hope this helps,
Old trucks and old trailers . . . a comfortable combination!
A-6E All Weather Attack Driver, BUFF
USN Aircraft Maintenance Officer, Ret.
'91 Suburban R-2500 w/BBC
'78 Honda GL-1000
'72 Triumph T-100R Daytona