The one thing I found is to really review where I was going, and whether I would ever be out of range of fuel stations at 300 miles. My FJ80 used to get about 240 miles per tank, and even in remote Baja, where I carried 2 NATO cans of fuel, I never really needed 'em.
In the most recent Mexico trip, the FJ45 was only once under 1/4 tank between stations, and we could have found fuel, albeit expensive, at a ranch or two we passed.
I guess what I'm saying is that extended tanks are usually pretty expensive, given the number of times they're really needed. If you're also looking at relocating the spare to the hood, add in the costs of bracing the hood and re-engineering the hood supports, and it's tough, at least for me, to really justify the expense, when a good NATO gascan will do the job for $40...add a high quality Expedition Exchange holder for another $100 or so (2 can carrier) and for under $200 you can have 10 extra gallons...
HTH
-H-