"-MIKE-" wrote in message news
There may be something you can't see in the pics. I added another photo
in the album to better show what's happening at that transition.
https://photos.app.goo.gl/8kX2emmUY3rLfTpi8

There are 2 layers of plywood subfloor, which is very common from this
time period. They put down 5/8", frame the house, then lay down 1/2" on
top. When I pulled up the old finished flooring, I also pulled up the
1/2" layer of plywood underneath it. The horizontal plane of that
little step is my horizontal guide for the new section to meet it.

Whatever I do to shim the new section to the old section will sort of
follow this little step and it will continue the plane of the old floor
the remaining 18" until it hits the wall and door threshold. That's all
I'm doing... continuing the flat plane of the floor to meet the wall.

Overall, in the photo it looks pretty much like I thought it did
conceptually... an ugly transition!

Me personally, I'd remove the sub-flooring down to the joists and use
tapered shims to make the joists transition as outlined previously. Then put
down the subflooring. If there is room perhaps use 3/4" ply for stiffness.
With the shimming done well a bit of sanding might be all you need to smooth
the transition. This assuming laminate padding/paper is used between the
sub-floor and finish floor to take care of minor variance (e.g., 1/32" or
less),

However, in the time it's
taken me to type all of these posts, I could probably have been done
with the wood route and had the old flooring put down. :-)

Yup... analysis paralysis is a common problem!