There are quite a few threads on the subject. The biggest thing isn't modifying the car, its finding the parts. Unless you just want to plunk down a big chunk of change for all new stuff.
You'll need:
A833 transmission (or 833 OD)
- e body shift rods
- shift mechanism w/shifter
- e body shift plate
Big or small block bell housing
- Shift fork and pivot
Flywheel
Clutch
4 speed pedals
4 speed transmission hump
- shift boot
- 4 speed carpet
Modified driveshaft
4 speed linkage
- Z bar
- clutch rod
- clutch adjuster
- Z bar mount bracket and ballstuds
- bushings for linkage
I'm sure there's some things I've forgotten. As far as modifying the car, the 4 speed pedals bolt in, no modifications to the firewall. You will have to cut a hole in the floor though, because the shift linkage and shifter have to come through the floor. The 4 speed hump covers the hole in the floor. You also have to weld the Z bar mount bracket to the frame. The other issue you might run into is that not all of the crankshafts were drilled deep enough for the input shaft and bushing. You can get around this by using a later roller bearing pilot bushing, as it fits into the outer register of the crank. But if that's the case you'll have to shorten the input shaft slightly. If you have a 727 auto the driveshaft is the same, but if you have a 904 the driveshaft length is different. Can't recall if its longer or shorter, I found a used one out of a 4 speed car.
The floor is the only sheetmetal that has to be cut, and the 4 speed hump and the z bar bracket are the only things that has to be welded on. The input shaft bit can be tricky if the crank isn't drilled for a 4 speed, but shortening the input shaft is the only cutting if it needs to be done. The rest should bolt it.
