The 904 behind my 318 was modified to work with the column shifter in my car. I want to unmodify it and change it back to a floor shift. I am looking for inexpensive shifters on craigslist, and I have the equipment to do any cutting or welding to the tranny tunnel. What linkage will I need? Can I chose between cable or mechanical linkage? What other pieces am I missing that I will need?

It all depends on the shifter you choose. Pretty much all after market shifters are cable operated and should come with proper brackets. Factory shifters are linkage, you would need to get proper one for your car. All that linkage that's on there now will be removed. You need both levers that should be on the selector shaft now. One is for kickdown other for gear selection.

A ratchet shifter would probably be your best bet. But I would stay with the better known manufacturers, like Hurst and B&M. As mentioned, all are cable operated. just have to route the cable so there aren't any sharp turns or kinks and keep it away from the exhaust pipes.

Personally I use a Hurst 1/4 stick with my manual reverse pattern trans in my race car. Many years ago, I had a Hurst Auto-Stick II in my street car.

If you retain the automatic shift features, you will need to retain the kickdown linkages and brackets.

To mount the shifter to the floor, I used a shallow piece of channel (Home depot has it.) Cut to the length of the shifter, I drilled and welded nuts to the underside to bolt the shifter down and easy removal. Then welded the channel to the floor in a comfortable spot.

I had a B&M ratchet shifter that the cable melted on me three times no matter how I routed it. Even wrapped it with heat wrap and anytime I got stuck in traffic it was usually fried. Went with a Lokar floor mount shifter that uses mechanical linkage. Check their website to see how linkage works. I added a boot and trim ring from a non-console four speed that worked pretty well. Not my intention but people now think my car is a four speed. Very happy with how it looks and functions.

I did it with some heim joints/allthread & a cheap floor shifter & there's not enough travel between gear positions so it's easy to move it several positions no matter how lightly I move it so I have to bump it with the palm of my hand & even then I have to go back & forth sevral times to get the gear position I want. ABSOLUTELY get one with detents that give you the same feel as the original colume shift did