Solution for the 5V vs 3.3V problem

Yes, it was just to get the headers defined, to show that 2 sets of headers could be made to fit next to each other - males down to the Due, and females up to shields.
I think I'll slide the 2nd set over some more to make room for signals to be buffered in the middle area and then go back out, especially in the double row area.
Are there many mega shields? Maybe just drop that area and make sure the 20 Uno IO + power + ICSP headers are supported. Need to beef up the power traces.

Crossroads
(with the system calling me "derekmjenkins" for some reason - hopefully just a test towards solving my week day log in problems)