Beefing up A500 overdrive
The A55/42RH and later 42RE are in basic form an A904 with an overdrive. Actually an A999, which was a heavy duty wide ratio 904 with a lockup converter. So a heavy duty rebuild would involve the same parts as the A999. Basically you want the 5 disk front clutch pack if not already equipped. Beyond that, a quality rebuild kit and a shift kit (Trans-go makes one, and would be my pick). Mopar went to plastic servo pistons in the 90's for some reason, and they were crap, really hurting the reputation of the TF, especially in the trucks. Fortunately earlier parts can be used, or Sonnax makes nice billet new ones. The later OD units from the electronic RE transmissions were supposedly better and are interchangeable, so if yours ever causes problems, just grab a later unit from a junkyard. Some later Diesel trucks (48RE) had a stronger planetary in the overdrive, overkill for your purpose. Converters were the same as A999 and came in several stall speeds. Mopar Performance even listed a few for a while. A good converter rebuilder should be able to set one up to suit your needs. Precision of New Hampton has a good reputation, and there are Mopar focused transmission shops for higher performance units. Later units used a different pump drive which was stronger but would require a corresponding pump.
So this is going behind a poly 318? Are you putting a poly in an A-body?!? Polys always had an A727 so an A518/46RH would be a more logical choice, stronger, easier to find, etc.
Keep in mind that the drums and clutches from an A727 based unit A518/46RH/47RH/48RH are not in any way compatible with the A500/42RH, so one can not simply "upgrade" to 46/47/48 spec.