I'm with David on this one - when you are printing en masse like this it is easiest to use a simple negative. I've gotten some 6x6 contact prints on the postcard paper that looked quite nice if you want to skip the enlarging process altogether.
We've had a lot of crops from the 6x6, but personally, nearly all of mine have been 6x6 uncropped. If it's centred it usually looks okay. My last one wasn't centred at all, but I think it still looked alright. Sometimes it looks okay if you do the old "French mount". I have a 4 blade easel and that helps a lot with this, even though I don't really know how to use it properly yet.
remember, though. It's a postcard exchange, not an Ilford Postcard Paper exchange.
Why not print 5x5-inch prints of the whole frame, and either dry-mount or spray mount to some art paper? Draw a line down the middle of the back, address it and voilà!