Once you've got the unit nestled in the frame where it's supposed to be (doglegs in the mounting flanges dictate location), grab a couple of C-clamps or quick-release "third-hand"-type clamps (right) to hold in place to ensure the proper fit. | 
Start drilling with the tank first; that way you can set it back in the frame and use it as a template to drill the 'rails.... | 
...(Also, it's probably best to wear eye protection, especially when drilling from below!) |
The welded flanges on the tank go against the underside of the lower framerail lip, so instead of simply using nuts and washers, POL came up with a full-length metal fastener strip to provide more strength. It will need to be bent uniform to the shape of the 'rail and flange. | 
Obviously, you don't want to bang up your shiny new tank right from the start, so either enlist an extra set of hands or, at the very least, use a stable floor jack when initially positioning the tank between the 'rails. | 
Once everything fits satisfactorily, pull the tank back out in order to transfer the mounting holes from the fastener strip. As an aid in locating and aligning the holes, grab a pencil or China marker and scribe the inner edge of the framerail on the flange. |
It might be a bit tricky to position the fastener strip while threading the bolts in from beneath, as there isn't a whole lot of room to work with, but it's not impossible. | 
First, verify that the fuel gauge functions properly. To do so, use a couple of jumper wires and tap into the factory sending unit wiring; with the ignition on, operate the float by hand to see if the gauge responds accurately. If not, you'll need to check the gauge itself and/or the wiring harness. | 
Don't forget to use lock washers on every bolt-that is, unless you're using a different type of fastener, like a button-head with a finish washer, in which case make sure to use plenty of Loctite. |