A friend of mine did one one his 70 Challenger. If i remember right, all he did was a little grinding of the trans case, a boss and a little off of a stiffening rib on the trans case. it went in without any trans tunnel modifications. He used a special trans mount for it, but that was about it.
if i remember a 42RH is based off a 904( A500 )? the one he used, was the A518 (46RH). I'll get a hold of him to clarify exactly what he did. it works great for him.