I was considering building a high altitude bridge/sky tunnel over the Corruption in the early game when I don't have good armor to survive the Devourer. How high above ground can it lunge?

I don't think Devourers spawn unless you're either in the Underground Corruption, or on/near the surface and considered to be in the Corruption. I would guess that all the higher you'd need to be to prevent Devourers from spawning is high enough that the ground is a little ways offscreen...though you'll likely still have to deal with some Eaters of Souls (which, for my money, are far more annoying and dangerous, just because there tends to be so many of them around at a time).
IIRC, Giant Worms are solely an underground creatures, so you should be safe from them even walking along on the surface.

Basically, if you're going to bother to build a skybridge over the Corruption, you might as well build it high enough that the game no longer considers you to be in the Corruption (just listen for the mucic change).