Back in my college days, I worked as a mechanic to put myself through school. The only engines that had significant end play in the cranks were those with a manual tranny setup. The pressure of releasing the clutch is taken up by the thrust bearings on the crank. the bearing will be worn on the back side, but unless the end play is causing problems on the timing gear end, you have little to worry about. You can add some shims behind the crank gear to take out the free play. I believe about .008" is about right on an older engine. Unless you are having problems that you can attribute to this, it is probably not worth fixing. If you are tearing down the engine for a rebuild, you will replace the bearings anyway. I would not tear down a good running engine for this, you can run years with that much end play and at the most have a little oil leak at the front seal.
The only real problems I ever saw was on an elderly Chevy dump truck with a 348. The crank had enough end play to cause timing chain issues (about .06"). I believe the REO engine uses two meshing gears, so you are probably OK.
