Maybe it would be a fun (although not exactly cheap) idea to retrofit an Ampere Altra motherboard from ASRock Rack into such an aluminum Mac Pro case?

After all, Apple already transitioned to the ARM architecture, so having such a case house a powerful ARM-based server would be kind of fitting.

How ATX-compatible are these cases? Knowing Apple, probably not very.

They definitely do not all have dual motherboards. I'm DD'ing an old 2010 5,1 and it's tray is single CPU. The "cheese grater" Mac Pro's were the 1,1, 2,1, 3,1, 4,1 and 5,1, they all came in the same case, the 2,1/3,1 and 4,1/5,1 were incredibly similar (respectively) internally with only tiny differences.

I think the earlier models like the 3,1 may have had only dual socket boards, but the 4,1 and the 5,1 both use a tray/riser card that holds the CPU/CPU's and RAM and they came in both single and dual CPU configurations.

3,1 motherboard:

4,1 and 5,1 style motherboard:

Trays:

Wow that sucks. Could always utilize the case as digger said. Those inner dimensions are critical though. Something tells me a 12" x 13" ssi-eeb serverboard may not fit. But then again maybe it will. Then it's another matter of getting it to coexist with a 5 1/4" device. Really probably not happening. I have a case like that. Has 1 maybe 2 5 1/4" bays, takes an SSI eeb sure enough. But not both. Or at least not sometimes . Intel with their damned heatsinks (long narrow ones). That's with the s5000 series anyway.