Stan wrote:
... In the end, I adjusted the gibs
for both the in and outfeed tables to the point now where, using a VERY
flat, thick and wide, 4ft starrett ruler, I have the in and outfeed
tables adjusted within microns of each other and the relationship to
the blade is practically perfect (I spent 5 or 6hrs doing it--I hate
giving up!). Now when I joint the two mating surfaces, I get a
1/64inch gap in the middle but the ends are flush. I should say again
that my boards are 5/4 hard maple, 5.5inches wide and about 57inches
long.

Now my question to you JOINTER EXPERTS: Is this gap simply inevitable,
given that my combined in and outfeed table length is about 47inches
and my stock is about 10inches longer than that or is there something
else I'm overlooking?
....

If the edge is concave, the outfeed table is just a tad high relative to
the knives. It should be possible to get essentially perfect contact on
a piece of that length. If the outfeed table is adjustable, the final
tweak should be nothing more than a nudge and test cut or two. As I
recall the Yorkcraft is a Delta clone so that should be the case.

Did you check that the infeed/outfeed tables are actually parallel to
each other, I presume? That also could be a cause if the ends droop
relative to each other.