Dear Bill and mid-60s Buick owners who are on a first name basis with too many TA-Performance employees!
Just spoke with Afton at T/A yesterday.
Shucks! I didn't know of an employee named Afton at TA-Performance . . .

I suppose that's a
good thing! 
Instead of having to call TA-Performance all the time to find out what was up with my trusty wagon's engine, these days enjoying my trusty wagon's engine is as easy as -
starting it up! . . . .

If you read the disclaimer, "Modifications may be necessary to fit in the 64-67 model years".
You might need to check with that
"forum down the hall," but I'm reasonably sure the mid-sump oil pan fits without issues. There were a lot of problems getting the engine into my wagon's engine bay, but nothing with the oil pan that I can remember.
Unfortunately, you may eventually find yourself between a rock and hard place on this one. The oil pan needs to be really true to avoid leaking at the seal.
Cheers, Edouard
P.S. A quick warning of
"upcoming attractions" . . . . . Are you thinking about headers or exhaust manifolds? Exhaust manifolds are okay I believe, but getting the steering column past the headers is really a tight squeeze.