How much are you willing to spend?
What's your knowledge of engine swaps?
Do you have another vehicle to drive while the 4runner is down?
These are the main questions you should answer first. If any of the answers don't fully support a swap, then do the head gasket and call it good.
While a 5VZ would be a good gain in power and reliability (ie., newer engine, less miles), to do it properly, you really should look at a cheap donor vehicle so you can get all the peripherals and wiring harness/ecu/gauges for the dash. I don't know all of what will transfer with the 3.4l, but I can tell you that its not for someone who has not done at least an engine swap! Its not like you pull one out and slap the other in and all is well! Mating the 4runner's wiring with the new harness and ecu requires a good amount of wiring skills and the ability to at least follow a schematic.
Not trying to talk you out of it, but you really need to understand what all goes into an engine swap. I've seen a lot a friends start a project, only to sell everything off dirt cheap after a few years of not getting anywhere with the swap. Either too much money was spent without getting close to finishing it, or they simply didn't have the technical skills to make it all go together.
And yes, I've done several now, including a 350 into my old '80 2wd longbed, a 6m HKS turbo engine into my '85 supra, several swapouts for similar engines, and right now working on a 1UZ swap into the '85 supra.
Just some caveats to think about. Even though you may have a complete engine, you may still need to tear it down and rebuild it. I got 2 1uz engines from a close friend for free, including all the wiring and 2 ecu's, but I knew I would need to rebuild the heads and clean everything up. I'm already 3 grand into the swap (including trans adapter kit for a W58 trans to 1UZ engine) and not quite finished buying parts just yet. I still have the whole exhaust system to rework and a hundred little parts/pieces that will add up quickly. You don't think about the little things until you're into it knee-deep! Stuff like spark plug wires, caps and rotors (the 1uz has two of each) and plugs already cost me 300 bucks!
If you are truly interested in a full swap, and you have at the very least, 4 grand to get the donor vehicle and a good set of metric tools, including impact wrench and a good torque wrench, then I'd say go for it! Once the vehicle's done and you're on the road, you'll be the happiest guy on the street, knowing YOU built that thing from the ground up!
And, if you have a lot more to spend, you can eliminate the issues with wiring by just buying a pre-made harness for the swap. That alone is worth the high cost for those who are afraid of wiring!
EDIT: Here's a good writeup for doing this exact swap:
http://www.4x4wire.com/toyota-sectio...ine-conversion
head gasket problems
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