My '85 K-10 Suburban with a 6.2 diesel has an external resiviour for the P/S pump--looks similar if not identical to the one I have off an '86 G-10 van I took the 305 out of to use in my '79 Bonanza C-10 back before I sold it..
I once put a Saginaw GM P/S pump on my 74 GMC plow truck,to power the plow cylinders after the Fisher pump crapped out one night during a storm..it worked,but lacked enough fluid capacity.--in a bind,I looked for anything I could use as a larger tank,and I saw a plastic gas tank off a Tecumseh engine I had off ,and I flipped it upside down,and joined the gas filler cap spout and the top of the P/S pump together with a hunk of radiator hose--the other end of the tank (used tp be the bottom,was now facing UP!),had a rubber grommet thing to hold a fuel line barbed nipple thing,I popped that out and screwed in a 1/4" NPT plug,it fit perfect..I had to bore a 1/16" hole in the pipe plug for a vent though..it looked goofy as hell,but it worked well,and thats all I cared about in the middle of a 18" snowstorm!..
Later on,I learned the "guts" from the Saginaw P/S pumps are identical to the one Fisher used on the "log" style plow pumps,and I simply swapped one of my spare GM pumps into the Fisher plow pump housing!..(Only can be done on the early ones that were removeable--later ones had the "log" brazed together,and were "disposeable")..
So,if you have an old snow plow pump hanging around,maybe that can be used on power steering as well,I dont see why not!..
