I guess I should have asked for some more details before posting the earlier advice. I made a few assumtions (yes, I know what those are) since you did not say that there were any other problems other than losing some coolant through the overflow. So here we go:
Is the car running hot / overheating? If you do not have a gauge, does it smell or act like it is too hot? If it is, check your coolant level, flow, thermostat performance and cap release pressure.
How old is the pressure cap and is it the correct pressure rating? I do do not know offhand what a stock 68 SB used, but I suspect it was in the 14Lb range. A garage or some parts stores should be able to check your cap for you.
Your cooling system will create pressure as it gets hot - this is normal. The radiator cap should hold in the water to a point - then the cap should release just enough pressure to keep the system from blowing. Of course when it does you may lose some coolant, thus the need for a coolant recovery system. It is also normal for the cap to release a small amount of pressure after shutting off the engine due to the pressure created by the engine "heatsoaking".
A couple of notes if you build your own recovery system: Make sure that you use a stiff overflow hose. The soft or flat ones will collapse during the recovery phase, so the coolant cannot return to the radiator. Also, I believe that recovery systems use an 18 Lb pressure cap.
Hope this helps-