Thank you all for great advice! I'm going to have to cut down a wrench
specifically for this spot, although I hope never to have to get in
there again!! It sounds like it's a pretty common trouble spot, and we
all have our own solutions to it :)

I soon realized that, by being stubborn, I was creating more work for
myself. The engine is actually out of the van right now, and this was
pretty much the last bolt required to get everything together again. I
ended up stripping out everything in the way of the rear-most piece of
tin, and removing that. I then had plenty of access, replaced the bolt
with a new stud, and snugged it all up together again. In the end it
took me about 30 minutes. What took longest was accepting the concept
that to go forwards, I must first go backwards :)

It takes some time to learn what parts have to go on in what order!
Even though I photographed much of the disassembly, I still found
myself having to pull parts off that I had just mounted, in order to
mount another part below/behind it. Ah well - it's been a lot of fun!
The engine is now ready to go into the van - just waiting for a day
off that coincides with my buddy's day off.

Cheers!
Mike.