Don't know whether your lathe has plain or rolling element bearings.? On a couple of ocasions, I've had to do the following to our 1943 14.5" SB:

Lift both main bearing caps and remove about .003" of shims.? Set aside carefully so they ultimately go back where they originated.? Replace the bearing caps and tighten down hard.? Do NOT remove too much shim, or the caps may distort.? Do not engage back gear.? You can now apply all the torque you can find a method to apply.? I had a 3' Stilson with another 3' scaffold pole extension and that didn't shift it without no.2 body applying lateral shocks over the female threaded extension of the backplate via a drift & 4 lb hand hammer.? On another occasion I applied a burning torch to the outside of the threaded part to shock expand it.

No harm has resulted from the above.? I only use the back gears for locking if my first jerk with the chuck key works.

Reassembly is a good opportunity to check the felts & bearing lift.

Ed