I'm running an autometer mechnical gauge on the XD with a 6 foot long tube. The running along the motor won't cause any false readings because its colder than the sensor anyway.
My firewall had a "spare" hole (ie: just big enough to fit the aluminium fitting thru from the inside - just behind the left rocker cover). I fitted a rubber grommet into the hole with a slit for the tubing to sit in so it doesn't rub and cut through.
Mechanicals are heaps better.
You can see all the round balck rubber grommets I have fitted to the firewall on the left and the numerous holes to choose from - although I've ripped most things out