The biggest I remember since I believe the 42RH is another name for the A500 transmission. All my Dakota Convertibles I've owned have had these. The biggest thing I can tell you is use some of the upgrades from a later 42RH. This will give you a few ideas here in this thread:
Found a few things on the web about A500 having an oiling problem (early models I think, but not sure) And something about cross drilling a main shaft for increase oiling ? Does anyone know what/if any oiling mods should/need to be done to an A500 while it's a part ?
www.forabodiesonly.com
The biggest failure point and what I always asked about the Dakota Convertibles is does the (4th gear) overdrive work. The first part on those trucks to own was the overdrive. I think two of them had the transmission rebuilt at lower mileage. If I remember correctly the earlier A500 (42RH) had an problem with keeping the overdrive gear properly lubricated if I remember correctly. I would add a shift kit too. On the other questions, I'm not quite there about the increase in cooler line size but, a safe bet would be to run a decent external cooler.