The steel shim thrust washers only began to be fitted to plain bearing engines when electric starters came into vogue so they were only intended to come into play for the short time the starter was used but even then it's necessary to make sure there's a little oil between the rubbing face. Normally there's quite a bit of clearance between the end of the crankpin and the backplate. This is easy to check just by pushing the crankshaft back with no prop fitted to get some idea of the clearance. As Bill said, there has to be a small amount of free play when cold to allow for differential expansion, .005" or .010" should be enough but more doesn't matter.

The problem with using it as a pusher engine is that the steel shim will give a very good seal that would most likely stop enough oil from covering the surface. What I would do is to take out the crankshaft then use a triangular file to cut a small groove in the front face where the shim rubs against it to provide a channel for oil to flow outwards from the crank bush. If you think about it, all bushed engines already use a plain steel thrust bearing when used normally. In that case it's the steel crankweb running against the inner end of the bush but there's a heck of a lot of oil floating around inside the crankcase