I use a 5hp Nissan longshaft on my Bristol 27. I've been very happy with it. Its given me good power motoring into 20kt headwinds. The only time I found it lacking was motoring straight into steady 35kt winds, gusting to 45kts, with 2-3 ft waves. Couldn't get more than a couple kts in those conditions. My boat displaces 6600#, and I get 5 kts at 1/2 power in calm conditions. I went with the smaller motor to keep weight down in the stern - B27s (and Ariels I think) tend to squat with too much weight back there. There was a thread on this a couple months ago you may want to look at. Suggest you mount the motor and try it out for the remainder of the season and see if it meets your needs.

Dan