1:20" is kind slow for .35 cal barrels, the Handi 357mag is 1:18¾" and the 35 Whelen is 1:16", both considered on the slow side. Cost for stubbing that I've seen is $250-$300 for the work plus the cost of the donor underlug and barrel, I just had a 6.5x55 done by Oregunsmithing for $300 plus the shilen blank I bought and a used barrel I included for the underlug. He also does reboring which I've had done in a couple custom chamberings, cost likely ends up about the same since you have to provide a barrel to start with too.
Tim