Northland shooting supply or packnor do remington drop in barrels with a savage type nut. If you want a conventional install: find a good gun smith and tell him what you want. Welcome aboard.
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.
We make our own smooth remmy nut and drop-in barrel system also. You can get it for trued and un-trued receivers. They use the sav smooth wrench.
Chris
Benchmark Barrels