You'd be crazy not to do this if you have no other easy means to get the Avios. Currently it will only cost you 143 quid. If this in effect promotes all 4 of you in CW, its the bargain of the decade!

Re the aircraft choice etc... Assuming you are a blue, unless you are willing to pay the seat reservation fees it will ultimately make little difference. There is no real difference between a 747 and 777 once you're in your seat (unless you do pay those fees for UD 747), so I would simply pick the service that is of most convenience to you.

One thing to note is that the peak arrival time into BOS seems to be between 7 and 9pm and in the summer queues can be long. You'll likely only be saving 30-40 mins or so max arriving outside of these times, so again I wouldn't avoid them if it unnecessarily inconveniences you more on take off side