Why would you want a heavy barrel on an SPS?
Most likely you will need a new stock and a new barrel depending on the profile of the new barrel.
You will most likely need the action trued when the smith puts the new barrel on. Course then you'll need a new trigger and some great optics. Probably need some kind of ballistic program for your iPhone. No iPhone or droid? New phone too.
Watch out this gets addictive but is a great way to destroy 1000 yard rocks.
i can vouch for benchmark producing good shooting barrels.
i have also seen quite a few standard bdl stocks with some pretty heavy barrels and still look pretty good. certainly varmit weight is doable.