If you are doing the plenum, just keep taking the rest off. The broken bolt means its rusted in pretty tough so you may need a flat head to GENTLY pry the intake up, just to break the seal. Also, douse the bolt in WD-40or PB blaster while you are taking the rest off.
Once you get the intake off, get yourself a propane torch or something hotter, and some WD-40 (no need to waste the good stuff on this part). Take the torch and heat the bolt up for 20-30 second, the douse it with WD-40 until it stops smoking. If it lights on fire, just let it burn out and don't heat it as much next time. Repeat this once more. Take a pair of vise grips and latch them on TIGHT. Just rock the vise grips back and forth a little at a time, if it moves, just keep turning it more and more on each pass. If it won't budge, go back to heating and cooling 3 or so times. Eventually it will come out.
If you do decide to weld a nut on, its imperative that you do two things. Disconnect the battery, and put the negative clamp on the HEAD, not the block or anything else. Putting the clamp in the intake ports is fine. If you clamp it to the block, current will flow partially through the headgasket, ground straps, and possibly bearings on the crank, con rod, and piston rings/piston.
I broke one bolt on mine and got it out real easy using the heat and cool method.
I'd also take a can of compressed air and blow all the crud out of the bolt holes too. You can also dab your finger in oil and touch the threads on the bolts when re-installing to make your life easier. Just remember, a torque wrench is only accurate when moving, if you stop, it will indicate a higher torque until the bolt starts moving again.
As long as you don't go all 300lb gorilla, you will be fine. Just use a couple touches of oil or assembly lube to hold the gaskets in place and double check everything once you set the intake back down, and you'll be fine.