[kludge]

Cool idea! Man, I love the old prog stuff. I was actually listening to Selling England by the Pound while reading this thread...

First and foremost, you need the SONG and ARRANGEMENT. The great prog bands were MASTER arrangers. Pick a few favorite tunes and just write out arrangement notes... what instruments when, keys and time signatures (if needed), comments on dynamics and mood shifts, complete lyrics, etc. You can learn a lot about how they were put together that way. Then do your composing along those models. I'd start with something small and contained - compare Yes' "Siberian Khatru", or King Crimson's "Epitaph", or somesuch. Don't try to write "Supper's Ready" right out of the gate!

Second, compose within your own chops and abilities. Check out the Flaming Lips album "The Soft Bulletin" for great ideas there. They aren't huge chops players, but they got the prog feel (and modernized it).

Now for production stuff, here are a bunch of ideas, many of which have already been mentioned:
1. Use only plate or chamber reverbs. I'd be a little more generous here and allow a tiny amount of room reverb on individual tracks, but just put a plate or a chamber on a send and make all the instruments share it.
2. Extensive use of mono and hard panning. Don't record instruments in stereo, generally.
3. Use characteristic instruments of the time... mellotrons and organs, dry thuddy toms, fuzz pedals.
4. Since you're relying on sampled drums, try adding live hand percussion for a more live feel.
5. Make a "desk". Use ONLY a single, "character" eq on every track. If it can't be done with something like Stillwell 1973, you live without. Compressors should be on busses and shared, and not many of 'em.
6. Limit the track count on your "desk" to 24, max. If you need more, render a submix and then delete the originals (you'll want to do this right away with the drums). Make commitments!
7. Don't ever edit timing on a track. If you can't play it perfectly, live with the imperfection.

I actually do a lot of this stuff as regular practice. I very nearly switched from Reaper to Propellerhead Record just for its forced traditional mixing desk and no-vst limitations. Ultimately, I stayed with Reaper largely because Record can't touch my NI B4 organ or Redline Reverb...