GTRV is an alternative, ColoradoCamperVans.com is a second. I'd contact CCV directly and ask Derrik, he's likely to chat structure and roofing, I know he's cut more than a few holes through roofs.
The "rule" under discussion visible above in any part of this section. Hopefully a clear distinction is made between asking how to copy a commercial builder's solution and information. Tread lightly, thinking you're right is not justification for rule violation.
However, mainly because you can ALWAYS ask the question, "Is this OK to ask?" first if you're concerned.
Closer to topic, SMB reinforces the sides after cutting out, it states or used to state on the website that the result is stronger than with a roof intact so it's not just a matter of measuring the hole anyway. From the outside it looks like the EB and RB Ford PH is the same, so you get an over-cab space on the EB with the left over van. I don't think this is structural, just easier since there is probably not enough demand for a full EB top mold.
Could cut a hole from X to Y doesn't mean should...
I wonder if there is some Ford supplied "body builder" codes you could get ahold of. I know things have been mentioned for the window cutouts.