Kanter, you are pretty damn close. I posted that from work and was working off my poor, aged and alzhiemer infected brain. When I got home tonight, I took the shock out and measured everything out. Turns out the shock is actually a 2.8. The marking was all dirty and scuffed out making the 8 look like a 5.
Anyway, here's what the Fox RC looks like off my 04 Fly:
11" eye to eye
Spring is 5 3/8" long
ID = 1.5"
450 x 2.8
So, if I go 500 x 2.75, I *should* be ok. The extra 50lbs should offset the .05" difference. Now the question is, I found a 450 x 2.75 Ti spring on eBay and I'm wondering if it's worth looking into. I'd really like to save a little weight, but I know I could pick up a 500lb spring for ALOT cheaper.