Gen III tranny options?

I'm still gathering parts for my build. Have a 6.1 Hemi and a 46RH for the 71 Duster but have started to hesitate.
Wouldn't a modern tranny with more gears be good? Time flies and it's hard to keep up with evolution!
When it comes to size I need to do some cutting and fabbing a new support to make the 46RH to fit. Then I could cut a little more and make room for something newer and better with more gears. Or is there a slimmer multi gear tranny out there?
Best shift-bang for the bucks? What options do we have?
Maybe I should add that cost is also a factor...