Fabricating fuel tanks is a very easy thing to do, and if you find the right fabricator, it shouldn't be more than a few hundred bucks. I worked for a company building high end sand cars for a long time and we custom built gas tanks for every single one of them.
If you or the shop have the welder required to weld aluminum, and can do nice welds, that is all you really need. Aluminum gas tanks don't have any special features or need any special tools or anything crazy. Just weld up a closed box or whatever shape you desire, then we would put like 15 PSI in the tank (do too much and it distorts), and use soapy water to test for any leaks.
There are two ways to set up the fuel pump. If you are like [MENTION=58830]JWaldz[/MENTION] and aren't worried about a fuel gauge, you simply add a small "sump" which is fancy talk for another small box on the bottom of the tank that will fill with gas. You need this so the pump is always supplied with fuel while the rest of the tank sloshes around. Then just run a simple in-line fuel pump you can get from almost any auto parts store (bosch makes the best ones from the parts stores) and an in-line filter and you are ready to party.
If you want to run a fuel gauge and do all that fun stuff, still simple. Just design your tank to incorperate the stock 4runner sending unit (or a different one if you want to get crazy) and then you will have full function of your gauges. I would really reccomend this route because if you are in a smog state, it will allow you to use all the factory smog crap that you need to keep the CEL off. It is a little more headache because you need to figure it all out, but is rather straight forward once you get it all sorted out and works just as good. I would also reccomend staying away from any of the aftermarket sending units and all that because agai nit doesn't play well with smog stuff or the factory gauges. We have done it when we put a fuel cell in a Ford Lightning, it's all possible, but it was a HUGE amount of work and headache trying to figure it all out.
Also one other note I leanerd from a previous Bronco project. If you are doing a rear ful tank with the factory sending unit/pump combo, build a hatch in the floor of your 4runner so you have access to the top of the tank without dropping the tank. The best way to do this is find a doner 4runner and cut out a section of floor that is larger than the hole you want to cut. Then all you have to do is lay the new floor over the hole and bolt it down. Put your carpet over it and it's hidden. But then if you want access to your tank, it's as easy and removing the pannel and you can change out anything. Also, have the tank steam cleaned by any randiator shop before install to get rid of any debree from building, it will save you a filter change down the road.
Here is an example I found on Google Images of what a custom tank could look like and this has the sump.
