Hello Nick and welcome to the forum. What a great boat you have, she looks capable of making any ocean crossing with ease. I had to look her up and the info that I base my recommendarion is here http://sailboatdata.com/viewrecord.asp?class_id=6225

I agree that the Beta 20 (BD722) will be the ideal engine for you but with your longer waterline and larger propeller you can expect to motor much faster than the Vanguard even if you have a bit more whetted surface.

I suggest the Beta 20 with 2.6:1 transmission. The prop calc says to expect 7.2Knots max if everything is right using a 16" x10" three blade. You should get almost the same max speed with a two blade prop but in general a two blade prop (16"x14" x2) gives very poor acceleration and drive when fitted in apeture on a full keel boat. I would speak to Fred Hutchison at PYI and confirm the pitch settings as my prop calc is generic and the flat blades of the Max-Prop behave a bit differently to the curved blades on a regular propeller.

 Have fun, Stanley