coil vs leaf,

[doowopper]

greetings.

i have a 1969 c20 that i bought new. i got it with coil springs on the rear, full well knowing that i shoulda gotten the leafs. the gears are 4:10 as i recall. i am think of putting a better ratio in as well as some sort of posi for pulling a small camping trailer in the desert.. according to a local shop there are no gears available for this rearend. i have the back half of my son's '74 c20. it has leaf springs and appears to have 3:73s. his frame web is 8" and mine is 6" so i didn't spend much time thinking about grafting it onto mine. one option is putting his rear axle under my truck. i will be putting an '03 5.3 ls with its factory trans in it. so here are my questions. is the change to leaf springs worth the trouble or can i add something to the coils to beef up the suspension a little? will the 4:10s work ok with the od trans? the truck will be my daily driver.
thanks in advance for any and all input.

roger