you can but you have to swap all the parts and they have to be the same model axle as well GM trucks used dana 60 rears under most of the C20 trucks but also the eaton HO52
so you gotta watch that
most of these dana axle equipped trucks were leaf spring rear, and most of the eaton axle equipped trucks were coil spring rear, but not always!
the C20 trucks were random like this these years from 1947 to 1972
all the C30 trucks were le;af spring and 40" apart leaf packs
so you would need to move the pads on the axle tube housings to make them 40" on the C20 axle
the C20 and less trucks were 42.5" apart
or you could remove and swap the leaf packs and hanger brackets and all from the C20 truck and put on the C30 chassis
or just swap the C30 body onto the C20 chassis and save all the headache of swapping all the C30 suspension and brakes and frame parts and then have a coil suspension truck most likely(if the 20 truck is a coil rear truck anyways) and most of the 20- trucks with dana rear were pow-r-lok locking diff axles which is great, the 30 trucks usually were just open diff standard nothing special and with the dual wheel package were an insane 5.13 ratio with no top speed at, not good for anything except in town driving.
you will want to swap entire rear axle regardless because you will run into axle shaft length issues among other things
i love the duallies especially the 72 older ones its too bad youre gonna do this to one id love to find me another dually 72 older chevy to drive i miss all the ones i had
theyre awesome trucks and easy to drive and the 6 cyl ones were great on gas and everything and ran forever
hmm
but good luck!
if you want to know any technical specs or RPO package specs or chassis dimensions info then let me know, i have and know them on these trucks, these were my first and always will be my first love. i grew up on these trucks and miss them all.
the rims were 17.5 and 19.5 available, other than 16.5
and you coild order a H110 axle under the dual wheel trucks and it was the eaton 11000lb axle and it had square axle tube housings and 10 bolt lugs only on the rear, the front were i think 6, i havent ever seen one in person yet but ive seen pics and all the literature and package info etc.
basically is like a 1.5 ton axle under a 1 ton