Ok, got out my first year .38 Spl SP101 (s/n 46) and a later .357 Mag SP101. Hhere are some measurements:
(.38 refers to the first year guns)
Cylinder diameter: .38 - 1.348" .357 - 1.348"
Cylinder length: .38 - 1.514" .357 - 1.581"
Frame opening front to back: .38 - 1.67" .357 - 1.74"
Frame opening top to bottom: .38 - 1.385 .357 - 1.398 (that 0.013" difference could be from machining the opening)
Barrel diameter: .38 - .682 .357 - .682
Barrel thickness (top to bottom): .38 - 1.138 .357 - 1.146 (again that difference could be from machining)
Using these dimensions, the .357 Mag cylinder of course will not fit the first year frame.
A .357 barrel should fit into the frame, and the .38 frame will handle the diameter of the barrel and has enough room to handle the extra 0.008" of the top to bottom size of the .357 barrel. The barrel should match up with the chambers because the position of the bore of the barrel is set by the threading of the frame.
That said, and knowing Ruger, I would guess the threads for the .38 barrel are probably the same as the .357 barrel. Ruger wants to keep things uniform if at all possible and why put different threads on the barrels, that costs time and money to make those changes. Remember the bore of a .357 barrel and a .38 barrel are the same, the only difference is what is roll marked on the outside of the barrel.
You may be able to find a 3" or 4" SP101 .38 or .357 barrel that has been removed from another gun to replace the barrel that is no longer any good. It will need to be installed by a competent gunsmith to make sure the barrel/cylinder gap is correct and that the front sight is timed correctly.
Hope this helps