I was looking at one of these 1991 and up c/3500 hd's the other day, thinking that a 60 series cab and clip would look really good on the chassis.
they are rated pretty heavy and would probably work for car hauling.
here is the wiki:
http://en.wikipedia.org/wiki/Chevrolet_C/K
imo you are probably opening a big can of worms trying to modify the og. 65 60 series chassis and suspension.
if that is what you want to do, I would look at "Z ing" the frame in the front, and maybe a the same in the rear.
a couple things I ran into w/ my 66 60 series:
insurance, I have to carry a separate commercial policy on it because of the weight.
registration, also because of the weight it costs me double that of my c/10.
wheels and tires, new tires are expensive, and if you have the old split rim style 9.00 20's you can't find a tire shop that will touch them, I was lucky and after a year of looking I found a rare set of 1965 22.5's that are not a split rim and will take a modern tire.
a couple times in that year I almost bought another 60 series just for better tires..
this yellow 61 is a 60 series that was placed on a 63-66 1 ton frame, and came out nice.