[midniteblues]

The steering box mounts too the frame so there will be no change there.
I'm not sure about the pitman arm from the 69 fitting to the later center link but that should be a easy fix using a later pitman.
If you do the swap I would suggest that you use the whole crossmember. If ya ask me its actually less work than removeing all the parts.
hard brake lines are ran different on the later trucks but will fit these yrs with minor mods.
It's not nessacary to use the later c member though.
One nice thing with the later suspension is most stores will still have the parts on the shelf with out ordering them in. Also usually there is a cheaper price differance with the later parts.
Some say the rubber bushings in the control arms give a nicer ride.

One thing I noticed with my 67 when I had a long water pump on the bbc is the clutch for the fan came really close for comfort too my 4 core rad. But also it was not a gen correct oe fan clutch either. I then used all short water pump stuff and all is great.
You might wanna do a search on the clam shell mounts and exhaust headers/manifolds fitments.

Also idk if you have leaf or coil rear suspension but pretty sure either way the 5 lug rear from the square will need the mounts modded and it will be wider.