You can just cut the tab off your cross member and move it. Weld it back in the right place. That's how I did it. Reuse your rubber mount (unless its wasted). I did a 400 Pontiac/TH400. Frankly, I wouldn't bother with the TH400 again. A well built TH350 or a 700R4 will handle what you are going to do to it on a street car. Heck, they are building 700's for diesel trucks that will tow in 4th gear and take a jillion ft-lbs of torque. TH350 will bolt in, the 700 will require the same thing to be done to the mounting tab (same length as a short tail 700). A 3 spd automatic in 2018. No thanks to that. plus the TH400 is a power leech. lots of guts to spin. If you don't need it, don't bother. I have about half a dozen of them on the shelf that will never see the bottom of a car. I'd even go 200 4R before a TH400. This is where people will tell me (and you) I'm batty.
You can build a fixture pretty easy to put a second hump in your cross member and move the tab. All you need is a couple chunks of 2x4 a decent piece of plywood and a few lag bolts. Cut the original in half, weld in the second hump and viola! Double hump, custom cross member with a trans mount where you want it. Bolts in and if you do it carefully, it will look factory. More or less.
I can send you a pic of my fixture if you want. I just did a T5 double hump for my GP. Re-used the mount tab and used some scrap steel I had laying around. I can send you a pic of that too if you need it.
