yes, if you take the head off, you should replace the gasket, and you can spray the new gasket with a gasket joining compound like hylomar, copper spray...etc for added insurance.
you can try re tensioning ( if it was never re tensioned after first warm up and cool down, that could be your problem right there)
loosen one nut at a time, starting from the inside nuts and work your way out to the edge.
no, you dont have to touch the base gasket, but just check the base nuts for tightness while your there.
gaskets are cheap, if its still leaking after a re torque, take the head off and check both the head and jugs for straightness (straight edge and feeler gauges) if all is good, get a new gasket spay it, tension it down in steps (10ft/lb, 15,20) working from inside out.
is your spacer plate under the jugs or under the head ?
is it leaking from the stud or head/jug joining area ?