I'm pretty much planning on the same thing... Moving plate for Y axis, entire gantry / bridge with Z and X axis moving up and down.
Reasons being:
1) need to fit into 3'x4' enclosure. (apartment machine) This gives me 32"x15"x9" travel using Thomson pre-made linear slides that I have.
2) sick of having a column in the way on either my Taig or the Grizzly at work. I have 9" Y travel on my Taig now, and can't use it all because I run into the column. As it is, I have (2) 1" spacer blocks hanging my spindle way out. <- not good!
3) if I move the bridge up and down instead of just the Z slide, I get my whole 9" of travel, unobstructed. If I hang my spindle off the moving plate of the slide instead, when it's all the way up and I have a tall part on there I'll hit the non-moving part of the slide that's hanging down. This way everything moves up and clears the whole workspace. Also hanging Z axis slide isn't taking the entire cutting force in X and Y directions, the bridge is.. Just seems to be more supported...
drawbacks:
1) I have the 2 big vertical members (towards the back) that I can still hit. Better than 1 central column in the way as I can get most things between them (~40" apart)..
2) lots of weight / mass moving up and down. I use a 30# gas strut on my Taig to neutrally balance the Z axis, and I intend to do the same on the bridge. That takes care of the weight for the steppers, but doesn't the inertia (or something?) still cause a problem with rapid up/down?
3) Lots of aluminum.

I already have 3 sections of 3060 (3"x6") 8020 aluminum extrusions for the uprights and the bridge. (my boss built a machine with a 6' gantry using a 3060 beam, with a 5HP spindle and Z axis hanging on it. Said it had almost no deflection. Woo! His had a 2.5" thick steel slab as a base though, slightly more stable...)
4) As drawn now, my spindle center is about 11" away from the edge of the upright supports. That's 2" for the Z axis slides (will have 2) which carry the 3" thick gantry, then 2" more for the X axis slide on the gantry, plus some mount plates. *NO* idea if / how much this will flex / have play. The Thomson slides are great, but that's a lot of stuff hanging out there.
I like the picture of that ancient green machine! At least that proves the concept is valid. Of course that's probably 3000lbs of cast iron instead of a couple hundred lbs of aluminum..... But still... Glimmer of hope.
As for other details, I plan on making the lower frame / base from 1030 extrusions (1"x3"), with a layer of 1/4" aluminum over a bed of the 1030 pieces. Then the 3060 uprights will bolt to the 1030 rails towards the back, with a "T" plate bolted down the side of them for extra holding. The machining surface will be .5" aluminum plate on top of a bed of 1030 extrusions, mounted to my Y axis slide plate in the center. The left and right ends will be supported by more linear rails, for a total of 4 rail supports in the Y direction.
I've been planning this for a long time now, without building much. Every time I find a bigger Thomson Superslide on eBay my design gets bigger / more complex. It started as 9"x9"x9". LOL Finally bought the huge 3060 sticks last week. I knew they were big, but WOW are they big.
I'll be following the thread for progress. I doubt I'll be making much for a while.
