One needs to look for pitman arm travel, measured at pivot ball, must be same or close to original; make sure a
PS unit turns the correct way; if the tie rod is in the front of the axle, or in the rear of the axle, different mounting provisions and locations will be required. Outside the frame will probably be easier. The '55 probably has no mounting provisions. Chevy(GMC) have had weak mounting attachments for years(cracked the frame) so make sure to brace the frame in anticipation. GM pickups have PS units that move the pitman arm(thus the ball) left and right or fore and aft, depending on 2WD or 4WD, so there is some flexibility there. Find room for a suitable steering shaft. Good luck.

'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.