What do I need to buy for my 727 swap?

Can I use the linkage from the 904 column shift on the 727 console shift?
Not easily, but you could pr0bably use most of the floor shift parts from the 71. Most A/B/E console shift cars used similiar pieces so they are not terribly hard to find. I suppose if you are very good with torches and welders, the column shift rod from the Z bar to the trans shift lever could be modified to work with a floor shift but it would require fabrication.

Can I use the 904 speedometer gear on the 727? Yes. BUT..You can't just "pop in" the speedo gear housing. It has to be aligned correctly to make the gears mesh

Can I use the kickdown linkage? most likely especially if you can get the levers from the 904. There is a possibility of very slight mods needed but should be able to work. Most of the problem will be with the carb bracket not being tall enough for a 4bbl carb. It will probably have to be bent upward. You may also have to bend the top rod from the carb. I have that exact same linkage on my 360 with a TQ

Can I use the oil dipstick tube from the 904 on the 727? Probably NOT the same

If I understand correctly, the 1968 tranmission used a single post neutral safety switch, and 69 and forward used three-post. Do I need to get a three-post switch and just hook up one of the cables to the car, to get it to work? the switches thread in and are interchangable. Use the correct single post switch for your 68 wiring.

If you can't do that, NOTE: Only one of the wires on the 69 up three prong switch (the center I think) is for neutral start. The other two are for the back up lights which are switched on the column (or shifter) on the 68. You could ignore the switch wires and hook up the neutral switch on the 3 position switch if you had to.

BTW the oil cooler lines from the 904 will work on the 727 as well. The transmission mount and crossmember are the same too. The 727 is longer so you will need a shorter driveshaft. Also the driveshaft yoke (that slides into the end of the tranny) will be different between a 904 and 727. Hope this helps!