392 in a 70 Duster
Sorry, I didn't get that crossmember measurement for you this weekend. I've spent most of my time since Thursday evening in bed. Now I have Pneumonia..... that should slowdown progress.
Chances are that you'll be getting close to that hole with your new trans hump because you'll probably wanna make a flange around the outside of the hump. It might be a waste to buy the whole side floorpan if you just end up needing a little patch.
I didn't have any decent pics of that side but I cut a pretty big hole out of a brand new AMD full floorpan!