42rh vs 46rh behind a 408

I'm toying with the idea of redoing the engine/transmission combo in my '68. Currently, it's got a 340/727, but I'll eventually be building a 408. For a transmission, I'm trying to decide between a 42rh and 46rh. I'm leaning in favor of a non-lockup 46rh so I could reuse the current Dynamic 9.5" converter that's in the 727 and have a few questions.

1. Is there any reason why using the current converter in a 46rh is a bad idea? For instance, will it build too much heat? I do have a good B&M plate cooler (lines also run through the radiator cooler).

2. Is there any reason to go with a 42rh (A500) instead of a 46rh (A518) besides weight and potentially a little less cutting to make room for it.

3. Lastly, do these OD units even hold up to ~450 hp and 450 tq in one of these little A-bodies?