Yeah, even magma will "evaporate" if it sits around too long at 1/7. But do the math - the number of tiles your map is X times Y, vs the volume (X times Y times Z) of your pipe.
If the result is not at least 2 deep, you'll first need to build a reservoir to "save up" enough to make it happen, emptying the pipe a couple times first into that if necessary. (Note that this "pour" will take a short while, and the pipe will do ~some~ re-filling, but not much.) But you'll then want to pump from the bottom of that reservoir too, so you could put it close to the pipe, and then once it's full, cut a new feeder channel to your existing pumps so they both flow in to the same source area without changing that part of your pumping system.
Then it's a matter of getting the magma to spread out - do some tests in your reservoir, observe and gauge. And then some water delivery one z-level above that - if you're going to collapse it, you don't care about perfect proportions, and can flood the magma a bit with a constant flow to get the job done without worrying about excess.
Here's the basics of standard obsidian farming - your situation will have to be customized, but I think most of the changes are in your favor since it's a "one time" thing.
http://www.bay12games.com/forum/index.php?topic=26805.0