My 68 was originally a column shift that someone converted to a 4-speed Muncie with a Hurst Competition Plus shifter. The problem is that the linkages are installed upside-down, making the shift pattern backwards. If I change them to be right side up, the linkage collides with the side of the transmission tunnel. Am I missing something? I have heard of a 4-speed tunnel hump, but I have never seen it on this site, and only NPD seems to carry it. Is this what I need?

I am just finishing the installation of 4 speed muncie conversion in my 68. I used the location of the 4 speed console shift plate to cut out the hole in the tunnel. I am not sure what you mean by 4 speed tunnel hump. It's pretty much just a hole in the right place for the stick to come through in the right position for the console. I had a regular hurst comp plus set-up but that meant one had to cut the console plate to stop it from hitting. I bit the bullet and ordered the correct one firebird one. I have never heard of someone setting up a muncie to shift backwards. I have a lot of pics here of the correct linkage set-up if that will help you at all.
http://www3.telus.net/public/68bird/transmission3.htm

I rechecked the part listing, and I realized that it is for 2nd-generation birds. I think I found the problem though. The previous owner did a really terrible job cutting the hole for the shifter, and he ended up patching a large part of the top and drivers side of the tunnel. The "repair" is what interferes with the linkage. I'll have to redo the patch properly sometime. I don't have a console (didn't like the look), so the rectangular stick should work just fine for me.

A friend went to a 5 spd richmond and it was bigger than the existing tunnel allowed , he just opened it up some more and made his own sheet metal cover thats a little larger ,then screwed it back onto the side of the tunnel....
just think outside the box!