Another cheaper option from LEDs is to check your current bulbs and see if they are #47 or #44 bulbs. If you pull a couple out of their sockets you can see one of those numbers stamped on the metal portion. These old machines came with #44's, but a #47 is the same size, it just runs cooler and outputs slightly less light. This in turn causes a lot less heat and a lot less stress on your power circuitry. In a dark room you're not going to notice the brightness difference, anyway.
I recently bought 200 #47 bulbs for ~$30 shipped. That would be more than enough (probably twice as many) to replace every playfield, general illumination, and back-box bulb in your machine. Also, I've strategically replaced some bulbs with LEDs where they don't stick out due to the brightness/color. For example, I've put colored LEDs in pop-bumpers to match the color theme for the game, and the opaque plastic hides the fact it is an LED vs regular bulb.
As a side note, some old pinballs don't "play nice" with LEDs. For example, the signals sent to the bulbs to turn off or on is technically pulsed and not constant-on. With regular bulbs you don't notice due to the natural fade-time the hot filament has when you remove power. But with LEDs they can flicker when placed in certain sockets. I don't recall if the early Williams SS pins like your Disco Fever have this issue, but I know my early Bally SS will flicker LEDs when placed in non-GI sockets. There's stuff you can do to fix this, but it is more than just a bulb swap.