Try this thread, lots of good info. Towards the end I believe you'll find some discussion on fuel usage. Are you sure it's an Alpha drive? They were paired together but Mercury learned the Alpha isn't up to the torque of the big block and quit putting them together. Everyone says don't accelerate too hard with that combination. Good luck!
This is only my opinion if you buy it I would find out what RPM you want to cruise at, then take the carb to a specialist have him set it up so your only running on the primary jets at that rpm when the secondaries kick in your fuel usage will sky rocket you should only need the secondaries to get up on plane unless you like big oil stocks.
