Most baby at boards will fit in a P3 time full ATX case, as the ISA/PCI slots will be on the right position. Above the board will be done empty space. Just make sure there are enough screw holes to put the spacers in place. You could create your own backplate from a blank one or just some aluminium tape and a piece of plastic.
Three power connector will be the biggest challenge here if you stick with pure S7. Lots of at Super7 boards allow for both at and ATX power connectors, along with the soft power button that comes with ATX form factor.

Super7 would actually be a good choice as the platform will support your CPU/graphics cards and allowed for future upgrades (larger hard drivers, AGP, even K6-3+) Performance may be pretty good compared to early S5/s7 chipsets. Just make sure it has 72p memory slots, or buy 64MB SDRAM for very cheap.

For super 7: clock the MMX to 83x3.5 for even better performance, most of them will do fine at just 2,7v or 2,8v 😀

Firstly I'd avoid eBay for better deals - but what other options you have depend on local conditions and I'm not familiar with situation around ABQ.

Then regarding the requirements, most will be offered by pretty much anything with at least two ISA and three PCI slots - but one stands out:

access to a 266

Pentium MMX 'Tillamook' 266MHz and 300MHz CPUs are mobile-only CPUs with good support (including cache with unmodded CPUs) on desktop boards being very . So if you want that, that determines the shortlist of boards you can use.

See this topic for options:
Tillamook 266MHz and working L2 cache?

Quick summary:
- Tillamook can only boot on subset of motherboards, mainly due to very low voltage requirement (and some BIOSs getting knickers into a twist with unknown CPU)
- Only very, very few boards (mainly with SiS and Via chipsets from non-premium brands) fully support CPU with working cache without modification.
- Modification doesn't help boards to boot, but if they do boot it ensures L2 cache works.
- Modification is simple, two little bridges.
- Impact of no L2 cache is limited, usually less than 3% (frequently less than 1.5%) in benchmarks

So go for one of the (many) boards in that topic and you should be good. Best overall (but not common) is ECS P5SJ-A - it ticks all your boxes (ATX, non-"Super" So7, 4x SIMM, 3x ISA, 4x PCI - and supports Tillamook fully without modification)

If you drop Tillamook requirement, you can easily find ATX Socket 7 board from Intel. Although I don't know if they can or can't boot with Tillamook, at least Intel motherboard can boot with unknown CPUs like AMD K6.

The Serpent Rider wrote on 2020-10-15, 12:45:

If you drop Tillamook requirement, you can easily find ATX Socket 7 board from Intel. Although I don't know if they can or can't boot with Tillamook, at least Intel motherboard can boot with unknown CPUs like AMD K6.

dionb wrote on 2020-10-15, 09:31:

Firstly I'd avoid eBay for better deals - but what other options you have depend on local conditions and I'm not familiar with s […]
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Firstly I'd avoid eBay for better deals - but what other options you have depend on local conditions and I'm not familiar with situation around ABQ.

Then regarding the requirements, most will be offered by pretty much anything with at least two ISA and three PCI slots - but one stands out:

access to a 266

Pentium MMX 'Tillamook' 266MHz and 300MHz CPUs are mobile-only CPUs with good support (including cache with unmodded CPUs) on desktop boards being very . So if you want that, that determines the shortlist of boards you can use.

Sorry, I meant to say 233. I’m not looking to make something that fast, since my Deschutes P2 is unlocked and able to make that sort of speed easily.
And the situation here locally is “here’s a single shop that has some stuff, but the prices are way too high to justify buying most, and one recycler that has old stuff, but they don’t really sell.” It’s why I am looking through eBay.

In that case, go for ATX Intel motherboard. Some already have OPL3 Yamaha sound chip integrated.

BetaC wrote on 2020-10-15, 16:42:

[...]

Sorry, I meant to say 233. I’m not looking to make something that fast, since my Deschutes P2 is unlocked and able to make that sort of speed easily.

Ah, that massively reduces complexity. I'd agree with an Intel board (such as a TC430HX or ML430HX) or this morning I saw an Asus XP55T2P4 (same spec as ML430HX basically) selling for EUR 25 on Amibay. With EDO RAM the i430HX is the chipset to look for. Most are prosumer/SOHO boards, so quality and compatibility tend to be high.

And the situation here locally is “here’s a single shop that has some stuff, but the prices are way too high to justify buying most, and one recycler that has old stuff, but they don’t really sell.” It’s why I am looking through eBay.

What about Craigslist?

dionb wrote on 2020-10-15, 19:44:

What about Craigslist?

It's usually a wasteland of toner cartridges and macbooks that are too highly priced to be worth looking at. Every once in a while you find something up there like an old Powercomputing Powerbase 180, but that's the exception. I will definitely be looking for a i430HX, thanks for the help guys.