It's going to be a bit of a hassle connecting in BOS. If coming from YTZ, you'll need to clear immigration in terminal E and then use a bus to get over to terminal C; I wouldn't want less than 90 minutes for the connection. I assume they'll have a way to check your bag at BOS on to the B6 flight after you get it at customs so you won't have to take your bag to the B6 counter in terminal C.