7050 sff fried motherboard after failed psu swap

I have a 7050 sff with 0nw6h5 motherboard and an antec TPQ-850 psu. The 7050 was in working condition before whatever I did wrong.

 I got the TPQ-850 used with an older pc gaming case. I removed everything out of the 7050sff case, trimmed the mounting plate out of it, and riveted it inside of the used gaming case. I did buy a 24pin to 6 pin adaptor from a well known auction site, which stated to work with 7050. After getting everything connected in the case, i powered up the psu. The pc did what I assume was a wakeup check feature of some sort, there was an arc between the rear usb housing and the gaming case, and thats all she wrote. Reinstalling original dell psu didnt change the outcome. zero signs of life after initial power with the tpq-850.

 I ordered 2 additional motherboards of the same configuration listed above, and am anxiously awaiting burning atleast 1 of them to pieces.

I found a general testing pinout for atx power supplies and verified that the tpq-850 has correct voltages at each wire of the 24 pin pack. I also verified the terminal wiring within the auction site adaptor are correct per the information found on this site.

Started chasing my tail while researching and figure I'd ask for help. Here are the caveats to my particular quagmire:

When fitting the board inside the new case, I removed the aluminized foam tape at the rear of the optiplex case that I trimmed, and therefore the usb ports etc were in contact with the metal case and this is where I observed the arc of demise.

The tpq-850, while not brand new and functioning, was in a dust bunny state, and had probably never been powered on in 10 years. And I didnt meter the 24 or clean the psu before cramming everything together and throwing the switch! I am aware of that being a oarticularly unwise thing to do, and it has been added to my wall of don'ts.

After scouring what little resources are avaiable, this forum being the most helpful, I've drawn a blank on the failure mode here, as well as the proper remedy. As stated, I have 2 more guinea pigs coming. I'm in for a pound on the 7050, the tpq-850 can be replaced, but internally and externally it's passing the smell test. I do need a psu in this range for the tesla gpu that i'll be using for photogrammetry and other hobbies.

Thank you for any insight or illuminations besides scrap the build.