The trickiest part is getting the appropriate tools into the pace between the tunnel wall and the transmission to get at the bolts that hold the tranny to the bell housing. It takes an articulated extension and some sort of hex adapter to fit into the bolts. Since you mention wanting to install the 4th gear switch, I'm assuming you plan to take the gearbox and the OD unit down together. (Which is what I would do even if I wanted to only work on the OD unit.) Make sure the transmission is in gear. It will make lining up the splines easier when you put it back. The OD unit can be a bit resistant to being removed from the gearbox. It helps if you make sure the OD is not engaged when you shut down the engine prior to removing the transmission. Some people also suggest that you momentarily move the car in reverse, shut down the engine, and then put the transmission back in gear - not the OD. You can find more info and some useful links at http://www.vclassics.com/

Bob S.
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'62 PV544 (B20, M41), '71 142E, '93 240 Classic Wagon.