surf1987,
If you have pictures, you might want to post them.
" . . a side mount spare tire carrier for a 1950 chevy 3100. . . it does not line up with the holes in the bedrailes or running boards?"
A 1953 side-mount spare-tire carrier (first year it was an option on an Advance-Design truck) should fit your 1950 bed (the 54/55 will not attach correctly to an earlier bed rail).
The side-mount spare tire-carrier does not attach to the running boards - it attaches to the driver's side bed rail and to the bed side. A dimpled fender is needed on a 1/2 ton Chevy bed.
1954/55 side-mount spare-tire carrier (from the Factory Assembly Manual)
Tim