Thank you for all your replies. Sorry for my delayed response!
It's hard to believe that no-one other than @Twin Jimmys has actually encountered this & done something about it - there are dozens of outfits offering this booster/dual-master kit and just say 'may need the crossmember' but it's clearly not the whole story
Thanks for your pics there, that was a clever solution.
OP has a torque tube transmission/rearend which calls for that crossmember and torque ball support mount.
I don't know how necessary that tiny rubber pad actually is. I drove a '52 for years, that unbeknownst to me, had no rubber left whatsoever on the mount and I drove it 4,000 miles at 60 miles an hour and never had any issues.
In '54, they changed to a beefier bellhousing/crossmember and eliminated the torque ball support mount and the bolt on crossmember.
If this were my truck, I would not install a crossmember and mount and drive it to see if there are any driveability issues.
Indeed, when I was installing the Brake Booster, I saw exact same, that my transmission rubber mount was like that too, no physical connection between the mount, with the rubber gone. So had been driving it like that.
So I just removed the original cross-member for the brake installation as it hadn't been doing anything anyway.
But thinking I should really do something to at least restore its original mount status.
I went ahead and purchased the cross-member - didn't yet purchase new transmission mount until I got the member to compare dimensions; (the cross-member was only $67 with free shipping with ~ $100 more for the mounting bracket/pad)
So - I compared the two and the new one effectively will put the center flange (where the transmission mount bolts) approx 2" lower.
Ergo a 2" spacer there at the mount would be required ....
- or I can try to mount a 1" spacer at each flange (if there's room for it to go higher) and a 1" spacer in the middle
