What's everyone actually doing with giant surplus caps?

I've got a similar haul sitting on my shelf, four 63V 10,000uF cans I pulled from a dead UPS someone was tossing, and mine ended up doing exactly what you're thinking about. I wired two of them across the output of my 13.8V bench supply for testing car amps, and it made a real difference. Before that, my supply would fold back and shut down every time an amp hit a big low note. With the caps there, the supply sees a steady average and the amp gets the punch it needs during the burst. Cheap fix for a problem I was having anyway. Mine weren't super old, but they'd sat unpowered for years, so I brought them up to voltage slowly through a resistor first and let them settle overnight. Two were fine, one got warm and went in the trash. That's the honest risk with surplus electrolytics, you'll probly lose one or two, but if they were as cheap as you say it doesn't matter. I'd skip the welder idea, personally. I looked into it with mine and decided the fun wasn't worth what happens if I got sloppy. The test bench supply is genuinely useful, it'll earn its shelf space.

I went down this road a few years back with a box of big cans I salvaged from an industrial drive, and honestly I'd pump the brakes on using them for anything audio. Mine measured fine on the meter but one of them had dried out inside, and it ran hot and bulged within a month of light duty. Electrolytics that have sat on a shelf for years lose their conditioning, and you can't tell by looking. The guy at your surplus shop probably has no idea how long those sat in a hot warehouse either. If I were you I'd test them before committing to any build. Charge one up through a lamp or a resistor first, let it sit, and see how fast it bleeds down. A healthy one holds charge for a good while. If it dumps itself in a minute or two, the guts are leaky and it's a paperweight.

For a use case, I ended up putting mine to work as a buffer on a 12V winch setup in the shed, which is about as forgiving an application as you can find. If a cap dies there, nothing expensive dies with it. I'd keep them away from anything where a failure takes out gear you care about, and honestly the spot welder idea is the one I'd shelve until you've got a fresh set bought for the job. Cheap surplus cans and that much stored energy is a bad pairing.

Back in the day we pulled these out of mainframe power supplies by the crate, and the old check was simple. Charge it through a plain light bulb in series. Bulb flashes bright then goes dark, cap is good. Bulb stays lit, you found your leaky one. Cheaper than any meter and it worked every time. I still catch myself doing it that way. I put a couple in the supply for my old ham rig years back and the hum on transmit cleaned right up. What's the actual uF rating on yours? That decides alot about where they fit.

Mine ended up in a treadmill motor conversion for a small metal lathe. Those motors run on rectified DC and without decent filtering the speed control would stutter and surge at low RPM. Two of the big cans in parallel across the DC side smoothed it out completley, and the lathe feels like a diferent machine now. The part that actually made it work was a bridge rectifier I pulled from a dead microwave, free and way more current capacity than I need. One thing to check first, a 24V transformer puts out closer to 34V once rectified, so make sure your caps are rated above whatever your supply actually delivers.

The lathe fix lines up with what mine did too, big cans on the DC side made an old drill press speed control behave way better. One thing I learned the hard way with cans that size, check whether the case itself is one of the connections before you mount them. I nearly screwed a pair down onto a grounded metal bracket before a friend spotted it. A couple plastic standoffs sorted it and they've been fine since.

Update: tried the light bulb trick on all of them last night. Most flashed and went dark like expected, but one can dims down and then keeps a faint glow in the filament. Not fully lit, not dark. Is that a borderline leaker I shoud set aside, or do some of these just behave that way when they're this big? Curious what the threshold is before you call one dead.