In order to keep the capacity, you'll need to look for a '60-'72 C40. Thsoe are the only years that GM offered IFS on a 1 1/2 ton truck. The currenty available aftermarket IFs will be far below capacity, as they are based on passenger car designs. You might be able to get away with the '73-'87 truck style, but you'll want to use the 1 ton version, preferrably from a Dually, but there will still be some dimisished capacity.
