How can I add a 240v 30 amp outlet to an old fuse box?

You are looking at a dead end with that old fuse box. Those pull-out blocks were ussually designed for the main disconnect and maybe one or two dedicated high-draw circuits like a range or a water heater, but they aren't expandable like a modern load center. There is no kit or adapter to safely add a double-pole circuit once those existing blocks are spoken for. Trying to tap into the bus bars directly or double-tapping a fuse holder is a massive fire hazard, especally with the mess of exposed copper you mentioned. Those Type S fustats are single-pole only, meaning they only provide 120v. To get 240v for your shop compressor, you need two hot legs that operate togather. In an old fuse setup, you'd need a dedicated pull-out block designed for a 240v load. If your current blocks are alredy utilized for the main and maybe an old appliance, you're physically out of space. And seeing neutrals and grounds jammed together on a single bar is a tell-tale sign the system was wired long before modern bonding and grounding standards were established. The only real path forward is a **full panel upgrade**. You have a 30 amp compressor, which is a heavy inductive load. Starting that motor creates a significant current spike that those old fuse connections might not handle well, especially if the internal contacts are pitted or oxidized from decades of use. A new 100 or 200 amp breaker panel gives you the physical room for a dedicated double-pole 30A breaker and finally separates your grounds and neutrals properly. Bite the bullet and get a modern panel installed. If you try to hack a solution into that cramped fuse box, you're asking for a localized fire or a blown main fuse that'll leave your whole house dark until you can hunt down an increasingly rare replacement block. It's the only way to get that shop equipment running without worrying about the house burning down every time the pressure switch kicks on.

That sounds like a huge job. I'm curious though, is there any way to just run a smaller subpanel off that old main box instead of replacing the whole thing? Or woud the old bus bars still be the bottleneck there? I’ve seen people talk about 'tapping' lines, but that sounds like exactly what you warned against. Also, those exposed copper wires the OP mentioned—is it common for those to just be left hanging in these old houses, or is that a sign someone was messing with it before? I’m trying to learn what’s 'normal' versus 'danger' in these ancient setups.

Trying to tap a subpanel off that old bus is just as risky as adding the circuit directly. If the main pull-out is alredy under heavy load, you're just stacking hazards. Those exposed copper wires are likely old grounds or abandoned circuits that someone never bothered to land beacuse the bar was too crowded. If you've got bare copper just floating around in a cramped box, it's a sign the previous owner was a hack. Don't add more load to a ticking time bomb; get the upgrade done so you can run that compressor without the lights dimming every time it cycles.