Ah, you're right, clearances are pretty tight back there. I did end up using a socket with the leads coming straight out, rather than 90 degrees, but I was able to trim down the tail on the little rubber boot and got just enough clearance with the center brake light on my '87. You might be on to something with the GM/Ford plastic 90 degree sockets, though, or something similar. maybe even from a motorcycle or trailer, since their lights are usually in much smaller housings? If you can get to one of the large NAPA stores (we have a big warehouse store around here, and then smaller satellite stores, too), they've got catalogs of stuff like that, and they've always let me browse for what might work.
Getting better, and by that I mean BRIGHTER, lights on the rear of my '74 Spider is something high on my list of things to do, as I'm always driving with one eye in the rear-view mirror, afraid of getting hit by someone in an SUV. You can't really put higher wattage bulbs in, as the heat will melt the lenses, and I've tried (several years ago) LED's, but they were weak and the light was very directional. My impression is that the newer LED's are quite a bit better, but I haven't tried any yet. Think about those big multi-LED flat-pane brake lights in a newer Cadillac, and that's what I'd like to have.
On my '87 I have the rear "fog" lights and original brake lights wired in parallel, and just installed a flasher module for the center brake light: Page 1