I've got LED packs like that too. The local Race Shop had a few sets for cheap too.
I also found an almost exact replacement bulb at the local electronic supply retailer. In my case it was Fry's (
www.frys.com) and though I didn't try to find them online, I found them in the store. I'd say it was easy, but I had to look at 30,375 different bulbs to find what I was looking for. Ugh.
I haven't attempted to replace them yet, but it looks like a matter of cutting the bare wires (the stock bulbs are permanently wired to the housings) and laying the new ones in. I like your idea of just mounting the LED's from the outside, but wanted to retain the stock plugs too.
Oh, and when you're trying to remove the housings from the dash RG75, be very careful. That plastic can be old and brittle, and those things are a VERY tight fit into the holes in the dash sometimes.
There may be a trick, which hopefully someone will pipe up with, but basically it's a squeeze thing. There are tabs on the top and bottom edges of the modules that snap in and hold them tightly to the dash.
In other words, easy to break 'em.
Paul