The E3D V6 has a shorter heater block than the one that comes stock on the printer. The hot-end itself fits correctly in the stock mount, but doesn't reach the heatbed even when the levelling screws are at their topmost position.
Ideally, you'd already have printed the shorter mount I linked to above before doing the swap; This was my "last resort" solution, since I didn't have access to another 3D printer to print the mount. Here's what I used:
Nylon spacers. These are needed under the springs+washers that come on the printer since they aren't tall enough when you use taller screws.
M3x30mm screws By stacking the stock washers and springs with the standoffs on these screws the heatbed will reach the V6. 30mm might be a bit long. If you have a fastener company near you bring a current screw and check. I think the ones I used where about 10mm more than stock.
As for installing the V6 itself, I just needed:
JST XHP connectors and pins to replace the connector on the V6's thermistor so it can connect to the printer's mainboard.
JST-VH pins for the heater (I reused the housing from the old hot-end.
You'll also need a crimping tool; I already had the Engineer PA-21 for this.