The cast iron are twice the price of tube headers but will outlast ten sets of tube headers. Are you selling it once it's done? If it's a keeper, then the choice is pretty easy unless you like swapping headers. They rot from the inside out and while I can't vouch for ceramic coating, that puts you into the cast iron price range.
Plumbing water heat is simple, beginner fabrication stuff, not makeshift. Compared to all the rest you are doing on your truck, this is nothing. You're modifying it - modify it a little more.
Now if you get a set of stainless steel tube headers with 3/8" thick flanges...
