I see where you didn't get the truck, but maybe the below info will help someone else in the future.
My 82 C10 had 3-on-the-tree when I bought it. The shifter linkage was very sloppy because many of the internal steering column parts were worn out. I can tell you from hours upon hours of searching, 75% of the parts are simply are not available. I ended up shimming the IDs of a few steering column rotating parts, which helped a lot.
However, the killer for me was a pot metal "shift bowl" that was cracked where the shift handle is secured by a pin. The crack was epoxied, but I didn't trust it. I never was able to find a new shift bowl anywhere in the country. I advertised my column for sale and pointed out its last remaining problem, and actually sold it to a guy who happened to have a new NOS shift bowl! What a great deal that was for him and me.
I converted to a TH350 trans, but had I kept the Saginaw I would have used a Hurst floor shifter and a "blank" shift bowl bowl for the column. I have that P/N somewhere if you need it.
As for the Saginaw 3-speed transmission itself, virtually every part is available for it. Simply google "Saginaw three speed transmission". There's are some reputable shops that sell professionally rebuilt ones for around $600, as I recall. Keep in mind there are 4 sets of gears ratios available. The 250 six uses a very low 3.50:1 first gear to get the truck moving, but that gear wouldn't live very long behind a V-8. The 3.11, 2.85, and 2.54 ratios are used behind the 292, 305, and 350.
Grooves on input shaft:
None
2.85
1.68
1.00
1 Groove
2.54
1.50
1.00
2 Grooves
3.11
1.84
1.00
3 Grooves
3.50
1.89
1.00