P.S. A mini-review of the 75 short barrel plus the rz tilt & shift adapter:
After a bit of time with the 75 short barrel and the t&s adapter, I am concluding that it's really not a good solution for architecture at all. The lens is roughly 35mm equiv., which makes it much too narrow for interiors, plus the t&s gizmo gives only ~1cm up and down, delivering almost no perspective correction for buildings small enough to fit into the field of view. A ~$300 crown graphic is far more capable, in terms of rendering interiors.
On a slightly more positive note, as a pano option, if you are open to stitching two frames together then you can probably get ~6x9 out of it using the rb 6x8 back. The image circle is probably substantial enough for that.
On another positive note, the tilt is sufficient for some creative effects at far focus. At near focus (i.e. tabletop) it's not really going to deliver anything special in that regard- you can mount just about any of the other lenses on the t&s gizmo if close focus and a bit of tilt is the only need. In this case the t&s gizmo just acts as a t&s macro spacer.
So for many of the purposes one associates with t&s, I think the 50 uld is far and away the best option... you would crop and/or correct perspective in post.
Concerning build, the 75 SB is well made, but it is a *big* piece. Think rb/rz fisheye but bigger! It lacks floating element correction and also has no simple way to deploy a filter (I guess I'd put one on the back). The lens is well made and has a lovely solid metal front cap, and like most rb/rz lenses it is built for battle. If launched from a cannon it would probably penetrate armor and still be usable for photography afterwards.
Bottom line for me: I am unconvinced by the 75 SB + t&s option; it is simply too narrow in application for my taste. The 180 SB may actually be more interesting, since it is closer to the field of view I'd want for portraits and perhaps landscapes. I find ~35mm equiv kinda blah.