Here's a shot of a stock trans tunnel and then a shot of my tunnel after I modified it for good clearance to the Tremec 5 speed. The current shape and clearance allows me to to lower the body down over the engine/trans at install time, but I was able to retain the horizontal portions of the original crossmember. I'm hoping that if I ever need to switch to an auto trans w/overdrive (old age comes faster than you expect) that it will fit in there without more modification other than the crossmember...
