If you choose to research the topic obsessively, as I have been doing lately, you'll find that there are about three locations where people have been mounting the fuel flow sensor, and everyone thinks the way they installed theirs is best. You can put it on the cabin floor downstream of the electric fuel pump, near the engine between the mechanical fuel pump and fuel injection servo, or in the line between the fuel servo and the fuel distribution spider on top of the crankcase. I accept the argument that your fuel flow readings will be more accurate the farther downstream your sensor is placed, but there are also vibratory and thermal factors to consider when mounting a somewhat delicate sensor in the harsh environment of an engine compartment. After a weekend of head-scratching, I couldn't settle on a location forward of the firewall that I found completely satisfactory, so I gave up and elected to mount my flow sensor on the cabin floor. I can live with the fact that I may get inaccurate readings when the electric fuel pump is turned on, but that will only occur for a few minutes per flight. There is theoretically an increased chance of vapor lock as a result of adding another restriction upstream of the mechanical fuel pump, but I don't know of any evidence of this actually happening in real life (and if it does, I can still turn on the electric pump). There's also the fact that I put the flow sensor on the cabin floor in my last airplane, and it seemed to work fine.
Anyway, end of rationalization section. I pulled all the fuel plumbing out of the fuselage – it's easier than it sounds: you only have to undo three flare nuts and four screws and the whole works lifts right out.
I cut the line that previously went from the electric fuel pump outlet up to the firewall, re-flared it, and installed the fuel flow sensor.
The sensor body is partially tucked under the mounting plate, so it won't hit the fuel pump housing. It's not attached to anything, just sandwiched between the mounting plate and the floor. I stuck some foam weatherstrip material to the bottom, to hopefully isolate it from exhaust vibrations coming up through the floor.
With the flow sensor spliced in, the fuel line runs up to the firewall as before. Notice that this location satisfies the manufacturer's recommendation to give the sensor 5" of straight line on both the inlet and outlet sides, which is much more difficult on the other side of the firewall.
The housing fits without hitting the sensor or any of the plumbing:
