Off hand I do not recall the specs of the stock cam but I do remember being absolutely AMAZED at how tame it was. IIRC the duration was not even above 190 at 0.05 and the total lift was something along the lines of 0.400 or less!
The problem is while a nice powerful but easily streetable cam recomendation is easily made by us, it is going to suffer with those stock heads. I STRONGLY urge you to at least do a port match on them, particularly the exhaust port. The exhaust has been a problem with Ford heads since the '50s, the 260/289/302s, even the HO-5.0s and GT40s sucked in that department. On the 65-67 289 it is around a 1-inch square!
Spend a few hours with a hand held grinder that you can rent and a set of bits, for under $100 you can really free up some power and open up a range of cams that wpouldn't have worked well with the stock heads. But to answer your question, a cam with total duration in the 250-260 range, and 0.05 duration in the 210-215 range will wake it up nicely, add in a 0.45 lift and you shouldn't have to worry about piston to valve clearance. A cam like that will provide a broad power band, very low and smooth idle, tons of vacuum for accessories, yet still show itself when you ask for juice. The cam specs in my sig are pretty mild by a racer's standards but will not work well at all with stock heads. I'll have a slight lope at idle but enough vacuum at least for brakes, plus a very flat torque curve, with 375 or more ft/lbs all the way from 2,000-6,000RPM. In my opinion the torque curve is what you should pay attention to for a street car or cruiser.
