The 46RH were based on the 727 Torqueflites and I believe the 42s are based on the lighter duty 904s. They didn't put 904s behind Big Blocks for a reason.

I used a 46RH in the 37 Dodge PU that I just got running. I'm impress enough with it I'm considering building another one to replace the 4 speed in my 57 Plymouth.

1957 Plymouth (Hemi, Dual Quads, A833 4 Speed 9 1/4 w 4.10) Sold
1937 Dodge Pickup (Hemi, 6X2 intake, 46RH, Dana 60 w 4.56) Sold
1968 Plymouth Valiant 2dr sedan (354 HEMI, 46RH w/4.30 gears)