I blew it!
I had all my maths with the floor at 74, but the center of the globe also at 74 rather than centered on the center of the pathway openings (76). Thus, OCD!!
Ugh!
I can only think of two options: Tear the whole thing apart, which will take forever; or lower the portal floor to 72 rather than 74. Option two will still be a lot of work, but a lot easier than starting over.
Sucketh!
I didin't figure out the error until I was done.
Edit: Or perhaps Turtle could just re-wipe.
As for wiping, this must be a new Nether seed. Nothing in there looks even remotely familiar.