PLT OF AN SMT ATX ACFT DSNDED OFF THE CORRECT ALT FOR VFR HDG RULE RESULTING IN A NMAC WITH ANOTHER ACFT GOING THE OPPOSITE DIRECTION.

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WHILE FLYING A 210 DEG MAGNETIC HDG ENRTE TO EWO VOR I OBSERVED ONE OF THESE CLOUD/RAIN SHAFTS ABOUT 5 MI AHEAD. IT EXTENDED ABOUT 3 MI FROM E TO W AND 1 MI FROM N TO S. THE DOWNDRAFTS PRODUCING THE PRECIPITATION HAD PULLED THE BASE OF THE CLOUD DOWN RAGGEDLY TO ABOUT 6000-6500 FT MSL AS EVIDENCED BY THE FACT THAT THE CLOUD BASE WAS SLIGHTLY OBSCURING THE HORIZON FROM MY PRESENT ALT. THE RAIN SHAFT; WHICH APPEARED TO BE A COMBINATION OF VIRGA AND LIGHT RAIN; WAS LIGHT ENOUGH THAT I COULD SEE THROUGH THE OTHER SIDE OF IT AND 3 TO 5 MI BEYOND. I BEGAN A DSCNT SO AS TO PASS WELL BENEATH THE BASE OF THE CLOUD AND EVENTUALLY ENDED UP AT 5500 FT MSL JUST AS I ENTERED THE AREA OF RAIN. THE PRECIPITATION ONLY LASTED ABOUT 20-30 SECONDS BEFORE I FLEW OUT THE SOUTHERN SIDE. ABOUT MIDWAY THROUGH THE RAIN SHAFT I LEANED OVER TO THE R SLIGHTLY TO BETTER OBSERVE THE ENG GAUGES WHILE I ADJUSTED THE PWR AND MIXTURE SETTINGS SINCE I HAD JUST DSNDED. AS I COMPLETED THIS TASK AND SAT BACK UPRIGHT IN MY SEAT I PASSED OUT OF THE RAIN SHAFT AND IMMEDIATELY BEGAN A SLOW PITCH UP TO INITIATE A CLB BACK TO THE APPROPRIATE VFR DIRECTION OF FLT ALT OF 6500 FT MSL. AS I DID THIS I OBSERVED AN SMT TWIN ENG AT ABOUT MY 2 O'CLOCK POS FLYING NEASTERLY ABOUT 200-500 FT BELOW AND ABOUT 1800-2400 FT HORIZLY FROM ME. HE WAS IN THE PROCESS OF INITIATING A SLIGHT L HAND TURN TOWARDS THE N POSSIBLY IN AN EFFORT TO INCREASE THE SEPARTATION BTWN US. BY THE TIME I OBSERVED THE SMT AND HAD IDENTED HIM AS A CONFLICT HE HAD ALREADY PASSED BENEATH ME AT MY 3 O'CLOCK POS AND WAS NO LONGER A THREAT.