42RH in 68 Dart advice/request

The cost to build the 46RH/RE for more horsepower is significantly more. The upgrades are basically the same as the 727.

The disadvantage of the a500 style trans is the lack of upgradeable parts (lockup input shaft namely). So similar to the 904, 500hp would be pushing it for the a500 in my opinion.

Slightly less geartrain power loss, but with the overdrive that will be the majority of power the loss with both transmissions.