Dear Ambassador Safari Tim,
Firstly, thank you so much for the VAP -- it's made my project much less difficult! And I'm glad you're still vintage.
Not positive it'll work in your situation, but I did just finish a complete sub-floor replacement with the shell "on" (of course, that's kind of technical as to whether or not it's really "on" when you take the whole subfloor out -- but it LOOKS like it's on). I have my belly pan trimmed back all the way around, not just open.
Anyway, although we (my disbelieving carpenter and I) started with exactly the piece you are trying to replace, we wound up having to take it out again after all the back pieces were in (to trim it some more), so it wound up being the next-to-last piece in, with a 2' strip just behind it being last. All my subfloor went in full-width (88")! It's amazing how flexible that body is when it's unattached from the floor!
I had had a welder double the center stringers in case we needed to cut the plywood in half, but it wasn't necessary. I'd give it a try full-width first. We had to angle every piece in under the street-side channel, then push the curbside wall outwards to drop the other side down, but it worked. For the front and rear curved sections, we also had to screw pieces of 2"x4" to the top temporarily, so we could GENTLY sledgehammer them slightly forward or sideways.
Oh, and we didn't have a usable old floor for a template either, had to make one using a cardboard HW heater box all taped together -- an 8-foot wide piece of cardboard would have made things a lot easier!