I managed to pick up a Sonnet 7100/8100 G3 upgrade, complete with cable and card. Let me tell you, going from a 601 to a 750 is a hell of a thing. If there's still interest in doing a reverse engineer on the card and cable, I'll see what I can do.
I was searching out a SCA to 50 pin adapter and found this on Amazon: https://www.amazon.com/CERRXIAN-Female-Adapter-Converter-Computer/dp/B09G27DNN4/
It claims to be a SCA to IDE adapter using a 50 pin header. I'm sure it's a typo but... was there a 50 pin IDE standard?
Yeah, I kinda figured that since they didn't respond to my initial query.
Realistically, these upgrades /can/ be used on these machines without the extender cables and frames, but it's one of those challenges that I figure is worth tilting at. I'm no EE by any means, but I'm sure there's folks...
I've really had good results with my SCSI2SD... aside from the microusb header shearing off because the structural supports weren't fully secured to the PCB. (ObLink: https://store.inertialcomputing.com/SCSI2SD-s/100.htm )
That's odd, the version I snagged off eBay must be a different revision where the cable seats on a pin header. (Don't have the board yet, still have to ask the seller about shipping status.)
Something also to look out for, the power barrel jack tends to get crusty with rust and debris; clean it out or replace it. I had some strange things go on with my machine and the plug itself was warm. Squirted some Deoxit on a wad of cotton and scrubbed it out, no problems since.
I think the primary thing for the chassis is just to get someone to sit down for a few days with one and model it up in 3D. The primary purpose of the chassis is to just put the PDS video card in the right place to fit through the NuBus slot (upside down) and do so stably.