RAM 545 RFE Trans into '69 Dart

This is interesting. I have been searching for a way around the either/or scenario of highway vs track gears. Cheapest way so far has been an o.d 833. Im guessing this trans is on the pricey side seeing how it's newer. Also said on wikipedia that there is a 6 speed version for diesels. How does it compare to an A 518 or similar performance wise? I know the 518 is known for overdrive failures.