last time i spoke to them was in 2020 i think, they answered my questions pretty quickly.
they're probably swamped right now for whatever reason. i remember advance induction became pretty bad at answering emails and getting back to you because they just didn't have enough people to keep up with very high demand. they haven't even updated their facebook page or website.
unfortunately, there aren't many options out there for GTO's besides having something custom made.
you could just order a driveshaft from their website outright...
Sometimes customer service is poor because the company is not good, and sometimes it is poor because they have a great product, they know it, and the world is beating a path to their door anyway. In DSS case, I believe it is the latter. Rather stunning though, cf prices have almost doubled in five years.
And those blue Sonnax flanges for the stock pinion yoke are readily available, so I'm sure they could make a driveshaft with that flange and your provided measurements.
I'm not using the OE tri-flange. IE sent a regular 1350 spicer yoke which I requested. So, in your case, I'm sure they would have a 9" Ford 1350 pinion yoke.