ill just say be careful doing anything you get off those horsepower guys. especially, in your case, dealing with the flatness of a cylinder head. using a piece of welded tubing as a "flat" surface to use as a sanding block? really? the tube is probably more bowed than the head. i'd rather use a stone to remove any raised burrs, nicks, scratches, stubborn gasket etc., they tend to leave the actual surface untouched while taking down the high spots, especially with harder materials (iron). we use 4" round stones are work to make sure our mill tables, fixtures, and actual parts actually sit flat on each other, theyre fairly inexpensive (i'm looking at one on msc right now for less than $25), and as long as you dont drop it, are really durable.
i just got my tfs head done up with new valves (bought for less than $200 off summit, chances are with stockers you can skip this) and took the heads to a local machine shop for the valve job, resizing the guides, and had the deck milled/surface for under $400, and this place was a bit higher than some of the other local places (although the have one of the better workmanship and timeline reputations around the stl metro, and were dropped off and picked up the next day). just doing the decks was 150, which would be well spent since youve already had sealing problems.