WHILE DSNDING IN IMC A SOLO B55 PLT DID NOT LEVEL AT THE ASSIGNED ALT BECAUSE OF A DISTR.

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I MISSED LEVELING OFF AT THE ATC ASSIGNED ALT DURING A VECTORED DSCNT. I WAS FLYING TO NH FROM VA ON AN IFR FLT PLAN AND DID NOT GET THE ROUTING I HAD REQUESTED OVER BETTER AREAS OF WX. WX ALONG MY ACTUAL FLT RTE WAS DETERIORATING AND MY 'STRIKEFINDER' AND NEXRAD WX WERE SHOWING THE STORMS AHEAD OF ME. I FELT SERIOUS WX WAS CLOSING IN SO I ASKED FOR DIVERSION HDGS THAT WOULD SAFELY TAKE ME E OF THE BUILDING WX. MY REQUEST TO GO E WAS DENIED BY ZBW WHO TOLD ME IF I COULDN'T CONTINUE ALONG THEIR PLANNED ROUTING THAT I SHOULD JUST LAND AT A DIFFERENT ARPT. THEY ALLOWED ME TO MAKE SOME HDG CHANGES TO TURN AROUND AND HANDED ME BACK TO APCH. APCH TOLD ME AVP WAS VFR AND I CHOSE TO LAND THERE TO WAIT OUT THE STORMS. I WAS GIVEN A VECTOR AND ASKED TO DSND FROM 7000 FT TO 6000 FT. I PUT THE AUTOPLT ON HDG AND DSCNT MODE (NO ALT PRE-SELECT AVAILABLE) AND STARTED TO LOOK FOR THE ENRTE APCH CHARTS; WHICH I HAD NOT ANTICIPATED NEEDING. THIS REQUIRED ME TO TURN AROUND IN MY SEAT AND REACH INTO MY FLT BAG FOR THOSE PLATES. WHILE LOOKING FOR THEM AND THEN GETTING OUT THE CORRECT PLATE I MISSED MY ALT AND DSNDED TO ABOUT 5400 FT WHEN THE CTLR ASKED ME TO CONFIRM I WAS AT 6000 FT. I TOLD HIM I HAD FLOWN THROUGH IT AND HE TOLD ME TO GO DOWN TO 5000 FT. THIS COULD HAVE BEEN VERY DANGEROUS HAD TERRAIN OR; OTHER FACTORS BEEN PRESENT. DISTRS WERE: THE TURB; THE DIVERSION ITSELF; THE NEED TO PULL PLATES FROM THE FLT BAG. HAD ATC BEEN MORE COOPERATIVE ORIGINALLY THE DIVERSION TO LAND WOULD NOT EVEN HAVE BEEN NECESSARY.