Your compression will be more than 10:1 with a zero deck height on the piston. You can't drop 1.5 points in compression with just a thick head gasket and an overbore. If the pistons are rated at 11.5:1 CR with an open chamber head you're going to be up around 11:1 easy. I'd suggest a set of 9:1 pistons with valve reliefs in them installed at zero deck height (top of the piston flush with the top of the bore), then you can mill the heads a bit and get it up to 9.5:1 which is great for as cruiser.
That 509 cam should not be showing 3hg of vacuum. Where are you taking your vacuum reading - from the vacuum port on the carb or at a manifold source? If you are taking it from the vacuum port on the carb then something is seriously wrong. You have a severe vacuum leak. Also, you definitely need to go through the carb. BTW what make and model carb is it? If it is an Edelbrock you'll definitely be having problems if your vacuum is that low because your metering rods will be operating improperly and causing your rich condition.
For a cruiser I always recommend this cam:
http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=720&sb=2
Its the Comp "Magnum Muscle" cam (grind # 268AH-10). The timing specs are:
222 degrees intake duration @ .050 (268 degrees advertised)
226 degrees exhaust duration @ .050 (276 degrees advertised)
110 degree lobe separation (for that nice choppy idle sound you are looking for)
lift on intake and exhaust is 0.464"
This is a nice upgrade from the stock cam, yet not so big as to be unpleasant in around town driving. If you installed a set of 1.6 ratio rockers your lift would go to 0.495" and you'd probably get about 8 more hp or so. This cam will produce around 12hg of vacuum or better I'm guessing. I have conteplated making the switch to this cam since I want more mpg on the highway. I figure 14 to 15 might be possible with this cam for my setup. It may not soundl like much of an increase (11.5 to 14) but it adds up quick when gas is at or above its present price.
