[Sharps40]

I'd mount it up inside the fenderwell. Use the bed to fender bolts as the route for the tubing by adding clamps for the filter and the tubeing. It'll be secure, high, plenty of ventilation and if you establish the open end of the filter pointing down,,,,, sort of an upside down "J" (like an upside down P-trap from under yer sink) no water will get in to the filter/down to the tank....

Modified followup.....

I believe what I had here previously was incorrect....looking at the 63 Chevy II that neck vent appears open to atmosphere....i.e. I don't think it ties back into a tank vent.

From some on line searching, those three lines on your tank should be supply, vent and return. Many of the GM tanks had some sort of antisyphon valve associated with the vent. In any event, it appears the best solution is venting the tank to a cannister as high as ya can. Evidently, the anit syphon valves are not avaialble as a repair item and unless the cannister is high, expanding fuel will run out the vent once the valve goes bad.