In my own relatively uneducated opinion, I would go a little easier on the square tube than nat_ster does. His use of custom hat channel was the first time I'd heard the idea. Square tube was the only method I'd heard of up to that point, and plenty of successful raises have been done with it.
However... I don't recall reading of a raise that involved bolted square tube. I only recall it being done with welds. For me that's the biggest concern here and it's because of the possibility of the tube crushing and losing its strength.
On my Blue Bird I've found a few places where connections are done with bolts. Most of them aren't truly structural though. There are a few that apparently are; the rear-most rib of the bus which is part of the body cantilever over the rear engine (the body is not supported by the frame out back) has this interesting clip piece inside the rib/hat channel.
With all that said: I agree with nat_ster that hat channel is superior to square tube. It's been several months now, but from memory I think the sheet metal shop charged me US$27 each for 10 foot lengths of 14 gauge galvanized steel hat channel made to my specification. I took them a sample of the Blue Bird channel and asked them to make a new channel to fit on top. We got the first pieces riveted into place Saturday, and it's beautiful. That shop did a great job on the channel. I show it here fastened with four 1/4" solid rivets; I haven't decided yet whether to do any more than that.