The cross beams actually came from a telecom rack used to hold really heavy rectifiers. I was planning on using angle iron but i came across the rack at work and cut it down. I haven't done any 10 gallon batches yet, but my next batch i make will be my first 10 gallon batch. of BM's centennial blonde, that stuff is great. But even for 5 gallon batches i fill my HLT up to 12 gallon point so i have left over hot water to flush my lines, pump, and CFC. I haven't seen even the slightest flex. I can't say enough about how happy i am with the wood frame.
As for the space between the Keg and wood frame, I went with 6 inches because like you i was concerned about the wood getting too hot. I probably could have gone down to 4 inches as the wood doesn't get hot at all.