Be careful with GM 3500 axles, there are at least two styles of steering arrangements I know of. One is the box is in front of the axle, drag link moves fore and aft, may be specific to 'P' models. Other is box is inside of frame, Pittman arm moves port and starboard, tie rod moves to starboard side of frame. One is not interchangeable with the other.

P-30 spindles(disc brake) suitable for our trucks have the box inside the frame, behind the axle, shaft goes through the frame, horizontal drag link moves fore and aft, hooked to curved steering arm, tie rod behind axle.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.