PLT OF A BEECH BE-A36 WAS ON THE WRONG ALT FOR A VFR HDG. HE ADJUSTED HIS HDG AFTER DEP CTLR ASKED HIS HDG.

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I DEPARTED MRY; VFR ABOUT 20 MINS AFTER SUNSET; ON A FLT RETURNING TO PAO. MY INTENDED RTE OF FLT WAS BACK VIA THE COAST; A FLT I HAD MADE MANY TIMES. BEFORE DEP; I SET THE ALT ALERT TO 4500 FT MSL; APPROPRIATE FOR A VFR FLT IN THE INTENDED DIRECTION AND ONE PROVIDING EXCELLENT TERRAIN CLRNC. DEPARTING MRY ON THE STANDARD VFR DEP (HDG 340 DEGS); MRY DEP CLRED ME TO RESUME MY OWN NAV PASSING THROUGH 2000 FT. I COULD SEE THAT SIGNIFICANT STRATUS HAD MOVED ON-SHORE ALONG THE INTENDED RTE OF FLT AND SO DECIDED TO ALTER THE RETURN FLT TO GO INLAND VIA GILROY AND THEN TRANSITION TO PAO VIA THE SJC CLASS C AIRSPACE. AS THE CLB CONTINUED; THE ALT ALERT CHIMED AND I LEVELED OFF AT 4500 FT MSL AND PROCEEDED. I VERIFIED FROM THE CHART THAT THE CRUISE ALT PROVIDED GOOD TERRAIN CLRNC -- THE HIGHEST TERRAIN WAS JUST LESS THAN 2000 FT MSL. I STARTED TO GET THE ATIS FOR PAO IN PREPARATION FOR A HDOF TO BAY APCH. SOMETIME LATER (PERHAPS 10 MINS); THE CTLR ASKED ME MY HDG AND I RESPONDED WITH '016 DEGS.' THE CTLR RESPONDED AND THEN ADVISED ME TO XFER TO BAY APCH. AS I CONTACTED BAY APCH; I REALIZED THAT I SHOULD HAVE BEEN AT AN ODD ALT PLUS 500 FT; APPROPRIATE FOR THE DIRECTION OF FLT. I CORRECTED BY ALTERING MY HDG TO 300 DEGS; A HDG THAT PUT ME ON COURSE FOR PAO. THIS SIT COULD HAVE BEEN AVOIDED BY USING THE CHANGE IN FLT PLAN TO TRIGGER A FULL ASSESSMENT OF ALL PARAMETERS OF THE FLT INCLUDING THE CRUISE ALT. THE COMBINATION OF NIGHT FLYING AND THE CHANGE OF PLAN CAUSED A RESOURCE CRUNCH LEADING TO THE ERROR.