I was quoted $4k in the Northeast to have the head gaskets swapped out in my 6.9L IDI (exact same thing as the 7.3) in my '87 E350. Needless to say, I decided to do it myself. I'm almost ready for engine re-install (you need to pull the engine to get the heads off in these diesel vans) and I've got probably 80 or so hours into the project (and I've got all the right tools for the job) and probably $600 in materials. It has been done right, but don't think this is a weekend project. My van has been out of commission all summer.
Good luck. Head gaskets in the E350 is about a crappy a wrenching job as there ever was.