Personally I'd swap in a newer Topkick/Kodiak axle front and rear from a 2wd so you have a newer and most popular bolt pattern and availability of running newer tires/rims along with better gearing for high speed. Not sure I'd be running the old 5/10 lug locking rims and bias over 100km/h. Not that I haven't run them that fast before, but I prefer the newer technology when going that fast lol
Any medium sized big rig axles can be used, it's just a matter of mounting them which isn't overly hard on a frame.
That could be a major rework in this case. The '60-'62 trucks have an ifs front end, not beam axle, even on the heavy trucks. The C40 is more or less a beefier version of the C10-C30 suspension.
