Andries:
When you get these bumpers built here's some recommendations since I've gone thru 2 bumpers now:
1. Reinforce the mount by using the side holes and run it all the way atleast 6-9 inches back. You will find 1 set of bolts that are right next to the front frame ends and another set about 6" beyond that. Utilize both as this will ensure that the bumper is secure (good for winching as the front frame butt ends are not strong enough.)
2. Winch location should sit as low as possible (the higher it sits up the more stress the bumper mounts will have to take on).
3. Shackle hangers should ideally sit right at the frame ends as that is the strongest point of reference. Anything higher and you risk the same problem as the winch.
4. Give at least a good 1" spacing between the bumper and the body panels as the front will flex and you don't want the bumper to eat your body panels/headlights etc.
5. Watch out for the A/C and the Power steering lines for the v8 and possibly the v6 engines as it sits very low to the driver side. There is a bracket to the outer side of the frame that you can unbolt. You will most likely have to remove that mount and bend it up as you will want to protect it w/ the bumper.
6. Make the front plate go down to the subframe right under the radiator. This way it protects the front of the radiator. The bottom of the radiator has a subframe that protects the radiator (new feature for the 4runner as the previous gen does not have this) and also doubles for the mount for the skid. I am currently developing a skid for the 4runner and will be using the same reference holes so if you can make sure it doesn't cover it up that would be greatly appreciated.
7. Keep the weight down to a minimum if possible as you don't want to incur too much additional weight compared to the stock bumper (unavoidable but then again you don't want a brick either). Factor in the winch as an additional 75-100# depending on configuration.
That's about it!

Good luck and go get em! Looks like it'll be a very sweet bumper!