I've always wanted to do this, and this is my first attempt.
I wanted that original Retro look so I made it appear original from the outside wherever I could.
I tested the rig in a standard case for a while, then worked on modifying the IBM XT Case. I had to chisel through a bunch of welds to remove the motherboard mounts and rear slots, then I used an angle grinder to widen the power supply hole and made room for the ATX IO Shield. Then I drilled out an ATX motherboard tray and the rear slot/io shield area from a crap case I didn't want and drilled/pop-riveted them into the XT case.
The optical drive is an apple laptop slot loading DVD with the floppy drive faceplace glued to it, and the HDD tray is an old 3.5 floppy drive to 5.25 adapter with it's front cut off, and the HDD faceplace glued to it (with LED attached for originality)
I added three fans (two side inlet, one rear exhaust) to increase cooling, and it seems so far to be okay! The smaller exhaust fan at the rear is a bit noisy under load, but the video card accounts for part of that anyway.
So far so good, I know it's a bit rough in places but I've done what I set out to :)
Thanks to all for the nice comments! :) I've really enjoyed sharing this experience with you all.