Does anyone have a blueprint where one can see the relative positions of the various headers on the Giga? (Mainly the distances between D8 - D7, D0 - D14, D21 - 5Vout on the transverse header, Vin - D76, D83 - A8, and the X and Y distances between D94 - GND on the transverse header.)
Hi @anwall. The headers around the outside edge of the GIGA R1 WiFi have the exact same positions as on the Mega 2560 board. So information you find for the Mega will also be applicable to the GIGA R1 WiFi. The popular Mega board has been out for many years more than the GIGA, and is open source hardware, so there is a lot more information available for the Mega.
Unfortunately the information available for the Mega won't provide information regarding the positions of the GIGA R1 WiFi board's "camera" headers, since those headers are unique to the GIGA.
Hello anwall,
From the official page of the Arduino Giga R1 you are able to download the step file of the board with all of the headers (or at least the holes for them)
Download the file and then make the measurement you need.
To do so, you could use free softwares such as eDrawings or FreeCAD.
If needed, you still can find some websites or softwares (maybe even 3D paint) to convert this file to another type (like stl) that could suit you.
Hope you'll find what you're looking for 
If you don't succeed in doing it, don't hesitate to ask for more help.
Hello anwall
Julián from Arduino here.
I think the more straightforward solution is to download the step file as mentioned and measure it yourself.
Team will take a look and will add additional drawings regarding connectors measurements but could take some time!
Thank you for your nice feedback!