I came across a great deal on a MAP Universal surge tank on eBay, so I decided to go that route.
It came complete with 2 walbro 267's, so that should be plenty of fuel.
The beauty of this one, and the smaller, horizontal Radium Surgetank for that matter, is that it fits under the aluminum plate/ cover that is installed instead of the spare tire wheel well, so it will effectively be outside of the cabin, and as such I won't need a separate box/ fire wall.
Like so: