I began with a gifted E21 shifter platform (Thanks, Jim!). I cut 12-15mm off of it and drilled new mounting holes, which should be about 97mm apart.
Then I bolted up new mounts with new hardware. It's important to note that I previously drilled and tapped the bosses for the mounts on the transmission. You can see here the finished piece. Note the mount/vibration dampener on the aft end of the platform. It's a mount from the air filter box from a 1984-85 318i.
I still had the E21 shifter in at this point, but a test fit enlightened me to the wear on the bushings that manifested itself in huge amounts of shifter play. I swapped in the E30 shifter, which has a considerably long throw. I may search for a shorter throw shifter later.
Here's everything in the car.
The shifter link is from a 1995-99 E36 M3/328i. As it turns out, the "knuckle" at the transmission end needs to be the stock E30 piece. I bought the E36 M3/328i part, but it is about 5mm shorter than the E30 part.
Here is the aft mount/vibration dampener.
It's just a piece of 3/4" bar stock with an M8 bolt through it. If I had taken more time, the hole might be centered. I guess I'll just have to live with the shame of less-than-perfect fabrication where no one will ever see it.
I also bolted up the guibo. As soon as I get another scrap of angle iron, I'll make the other bracket to relocate the center support bearing.
The shifter is about 1/4" forward of the center of the shifter hole. Now I just need to find or make a shifter knob.