Depends on the reason the headspace is off. If the lugs are setback on the bolt, I would probably buy a different bolt. This may or may not require setting the barrel back to have correct headspace.
If it is a bad chamber, it could require trimming more than one thread off to clean up the chamber, if it can be done at all. You're right it really isn't a big deal to inlet the stock for the boss if the barrel can be set back, just money at the shop rate of the Smith you'll be using if you don't do it yourself. I'm sure most barrel makes have the 721 contour with rear sight boss programmed already so it wouldn't be a big deal to match the original barrel, once installed a head spaced then the Smith can cut the dovetail for the rear sight in the boss.
Until we know the exactly what is wrong it the headspace we can only speculate what will be involved in fixing the rifle. I wish the OP had taken some pictures of the ammunition he shot. The headspace problem could be a lot of different things to include a re-chambered barrel into something like a .30 Gibbs to where the shoulder has been moved significantly forward.