Sorry, hadn't seen the PM yet...and yeah, no problem on sharing the measurements of the tunnel. Actually, we "massaged" the plans slightly to give extra room for this bellhousing & trans. As you know, the T-5 is a great (and essentially bulletproof, in this application) unit, and very light weight.
Anyway, my trans tunnel is 11" wide, inside, at the firewall (so, 13" wide to the outside of the two 1"X1" square-tube verticals framing the mouth of the tunnel) tapering to 6" wide inside at the back of the trans tunnel; 13 1/2" high at the front (inside) tapering to 8" high at the back of the trans tunnel (roughly where the shifter will be, I mean); and 23 1/2" long (inside measurement) from the back of the vertical firewall tubes to the end of the tunnel (again, where the shifter will be). I hope this makes sense....
With this trans, the shifter usually ends up under the dashboard somewhere. I came up with a shifter extension (based on several different designs I've seen) that moves it back 6". It's made out of scrap steel pieces (i.e., no bought or pre-made pieces required), and is slick enought that the trans shifts better with it than without it. Let me know if you need to make one of these - I made CAD drawings that I can post, and I have photos of its construction as well.