Not hard at all mate....take a tap off a pipe, run it through the bulkhead into the cabin (there is a convenient grommet behind the drivers side footwell carpet), run the piping to the guage and done, I got mine from R-Spec and it's electronic rather than mechanical so it has wiring rather that hose up the side of the A-Pillar but the theory is the same.
Word of advice though...the nylon tubing you get with them is a right bastard, I got some silicon vacuum tubing as it is more pliable to fit onto the T-pieces etc.
I'll pm you a link to a site that is all about the S/V40 and there are plenty of guides in there on how to do it
Took me over all about an hour to do including the wiring and this was at 9pm in the rain lol
One more word of advice, there is no direct fit A-Pillar mounting pods for the S40, either a universal one needs to be cut down to fit then attached to the pillar or you can use mounting pods and attach to the dash etc...the choice is yours as to where you put it. I was thinking of moving mine to an air vent and just using a hole cutter to fir it