I have 602s and the 600 center with the Denon 3803. A good match in my opinion. But for the 604s, I'd probably go for more power. The 3805 would work, but the full potential of these speakers really needs an amp with more oomph.
You, of course, realize that a 120" diag 4:3 screen stands 6' tall? With a center speaker "close" to listening height of around 2.5-3.5', your room would need 9' ceilings. And you'd probably need a >$10k scaler to watch anything but HDTV and DVDs on it (won't find me watching standard TV or VCR tape on my 94" 16x9 screen). I see 4:3 as a step backward, but others obviously have a different opinion. Even games will be increasingly produced in 16x9 format.
Keystoning shouldn't be much of a problem, if any. With an HP screen, you want the pj to be closer to the middle of the screen to get the full gain (as you mention). Ceiling mounting isn't a great option as you sacrifice much or all of the gain - unless you extend the mount down to the point where it's close to your head level. But then you don't need to ceiling-mount it, build the shelf or stand instead (for much cheaper than a ceiling mount!).
You can test if...
Sounds like a plan, although I think I'd just cut off the extral 11" of material before mounting. Give it a better finished look when masked off. Should be easy to cut with a utility knife and a straight edge.
I have no experience with the adhesives, etc, in the first sections of your plan, so hopefully someone else will answer those.
Masking with a velvet or velveteen ($7 a yard at Michael's, WalMart, etc) works very well. I used a 3M spray adhesive to attach to...
How about building (or buying) a stand on which to place the projector? Say at 54-60" from the floor (above your head when you sit). It sounds like you're sitting behind the pj now? You must be WAY back if so. If I were in your situation, I'd sit in front of the pj and place it on a stand behind my head. With your 115" screen, you'll probably still be sitting 14-16' away which should be fine (throw for that size is around 17' for the X1).
I love the Taxi series of movies. I work at a French company and a friend lent me the first movie (on VHS, alas). Great stuff. If I can get my HTPC to connect through my new wireless setup, I'll give this one a whirl.
While I'll agree with the dalite samples being (far) too small, I have to say that Stewart is a stand-out here. Their samples are 2'x3' - certainly big enough to assess their qualities (which, in my experience are excellent).
And I, too, would like to second this idea. Say $10 for a 2'x3' piece sent out by USPS or another reasonable carrier shouldn't be too much to ask.