As for the frame connectors, on my '64 I used 2x2 and cut a window in the rear rails.
Drilled a couple holes in the sides of those rails.
Slid the 2x2 into the rear rails,
Welded the perimeter & at holes.
Used the usual plate at the front.
On that car, part of the rear footwells touched the 2x2 but wasn't an issue.
If you do that method, you'll know about any interference before replacing the floor.
Obviously, they stiffen the structure while floor is out.
I've heard that the connectors that weld along the floors create a problem with the E-brake cable, but a tube for the cable would solve that.