Cut part of the back out for IO. Make the sure standoffs line up with card slots. Plug in the switch wires to the ATX board's pwr_sw header.

NOt sure I fully understand what you are saying....the case has the switch on it. The PSU is an ATX PSU with power into the board and into the components....not sure what I'd need to do to get the switch attached to the pwr switch header

Can you elaborate or share photos? ATX cases have power switches. This AT one does not have the pair of 0.1" machine pin receptacles?

You don't. An AT power switch is not suitable for use as an ATX power switch. ATX soft power works by momentarily bridging the 2 pins to power on the system, while AT power switches directly connect and break the incoming mains poewer. If you were to use the AT switch for the soft power you would see the system turn on and after 4 seconds power down.

If your case has a reset switch you can use that instead, but the AT power switch is not suitable.

So it's a SPST toggle switch which has to be replaced with a momentary device.

There's probably a part which is the same style of switch but the mechanical is momentary. You'd swap it out then.

I'd advise against it. I DID Case-mod an AT into an ATX back when AT cases were plentiful - I regret it and wish I had that case now for my 486. Oh well.

What it entails to put an ATX system into an AT case? Well, this is what I had to do to make this work....and it still requires an ATX case to work....

BEFORE

- hacksaw out the backplane, that meant removing 1/3rd of the expansion slots, and cutting a HUGE rectangular hole in the back of the case where the I/O sheild goes.
- hacksaw out the motherboard tray and backplane exp/slots/IO Sheild panel to put in the back of the case.
- In my case I had an AT PSU, so I had to wire up ATX guts INTO the AT PSU casing, and then replace the clicky paddle switch with a push button from Radio Shack
- wire a new header to that button to go to the motherboard
- Paint the back of the chassis so it did not look like a mess with steel and coke can aluminum all mixed
- Cut holes ABOVE the I/O sheild into the original steel case for the cooling system for the CPU.
- Drill holes in the bottom of the case in strategic locations to bolt the motherboard tray in place, ditto the backplane as well
- cut the drive cage in the AT case so the 24 pin PSU header would clear the drive cage (had to use that slot for a HDD anyway)
- use 2 old Hardcard Frames to accomodate my additional hard disks
- rewire the LED light connectors to fit an ATX motherboard header instead of an AT one

AFTER

After all that, I used this system for 10+ years before the case finally self destructed, because I had to keep modifying it to make changes/upgardes over the years. It had 2 repaints too.

If you really want a "retro" looking modern system you can do what I did and find an older InWin case, this is my current Core2 system, it's mATX, but InWin and Antec and the others all made some beige and white (that can be UV faded to beige) ATX cases.

Those look like XT