Had a marlin 444, but never shot any of the leverution ammo. I eventually traded because it was a little redundant for me. I have, and love, two 45/70's. You can pretty much handload a 45/70 to similar specs of the 444. However, I have heard that the leverution cartridges are pretty nifty. The 444 is actually pretty high velocity and flat shooting for a lever gun, particularly with the new heaters they are putting out for it.
Don, if you've got a nice, custom made, straight shooting 30-06, why rebarrel it? You could always spend a few hundred on a ruger or remington 30-06 and rebarrel it with little work at all if you really want the 338-06. No reason to change or mess with an old favorite that shoots where you point it, even if it is just an odd six (just kidding, that's a cliche that never was really that witty). I may of said this earlier, but I rebarreled a ruger mark II -06 to a 338-06. I'm very fond of it, but I am considering trading it, with a couple other rifles, for a new 375 H&H mauser. Never had a mauser, but always wanted one. Probably end up rebarreling it etc., as I have a hard time leaving anything alone. I also have a hard time keeping the same guns in the safe for too long. I've got the bug pretty bad, but I, and my wife, have learned to just roll with it.