I would say that depending on how much work you intend to put into the gun, getting a Hastings cantilever slug barrel is your cheapest , and probably close to the most accurate alternative (aside from the Savage)
Getting your barrel pinned is certainly a possibility however the quality of the pinning job will really effect the overall accuracy. Get some crappy gunsmith work and you may not realize the full potential of your slug gun.
I have a Hastings fully rifled cantilever on my Browning BPS and have been very happy with the accuracy and have been totally impressed with it's ability to hold zero. I have had the barrel/scope combo on and off the gun over the last 5 years and don't think I have had to adjust it more than one or two times.