Joe,
I wish I could keep everything I've made or owned, but I couldn't afford the insurance, let alone the storage building and the upkeep...
Nice as it is, the next one will be even better, built with enough love and passion, the pain of selling the old one will soon fade...
Dennis,
I bought stainless steel piano hinge online.
Some I think from E bay, and the rest from Austin Hardware.
65 inches is a pretty long piece.
Quality 13 ply will go a long way toward stability, though just as important is sealing all sides evenly.
That is, if you apply 4 coats of whatever on the outside, you should also have 4 coats, not 3 or 5 on the inside.
The edges aren't as critical, and in fact I'd give them several extra coats as end grain will absorb more material.
With all those thoughts, If I had a door that tall and wasn't using multiple latch points, I'd put some sort of stiffener on it, either inside or out.
The full length piano hinge will take care of the hinge side, and the latch will take care of the center on the other, so the stiffener should be placed vertically near the latch, stopping just short of the top and bottom of the door
As the 3/4" door, against the interior stop will require spacing out to latch, running a 1 x 2, or 3 just inside where it will clear the stop board, will provide you with a spacer and stiffener that looks like it belongs.
If that doesn't make sense, look at this picture and study the door latch.
The latch is attached to a small block of wood, acting as a spacer.
Instead of using a block, use a full length board installed vertically next to the interior stop, with a 1/4" or so space between.
Rob