AN AC69 PLT WHILE ENCOUNTERING WX AND TURB; RECEIVES AN ALT CLRNC AND READS IT BACK; BUT CLBS TO ANOTHER ALT.

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CHAIN OF EVENTS. FLYING INTO GUC WHILE ENCOUNTERING SEVERE TURB I HEARD OTHER ACFT RPTING SEVERE TURB AND ASKING FOR BLOCK ALT; SAYING THEY WERE BALLOONING 1000 FT OR MORE. I ONLY HAD 8 HRS IN THE AC69; FLYING IT AS A SINGLE PLT IN TURB; ICING; IMC AND ENTERING A HOLDING PATTERN TO CLB IN AND GOING THRU CHKLISTS AT THE SAME TIME. EVEN FOR AN 11000 HR PLT IT IS A HARD WORKLOAD. ATC THEN GAVE ME A NEW ALT OF FL200 WHICH I REPEATED BACK AS FL200 THEY TELL ME; AND THEY HAVE THE RECORDINGS SO I'M SURE THEY'RE RIGHT. HOWEVER; I ROLLED IN FL220 IN MY ALT ALERT AND CONTINUED TO CLB IN HOLD TO FL125 FROM THEN TO FL220. I FLEW FOR ABOUT 50 MILES AT FL220 WHEN THEY GAVE ME A 40 DEG TURN FOR TFC AND ASKED ME MY ASSIGNED ALT. THEY TOLD ME TO CLB TO FL250 AND I WAS ASSIGNED FL200. THEN ATC SAID THEY HAD LOST RADAR CONTACT WITH A C130 BECAUSE I WAS AT WRONG ALT. CLEARLY MY FAULT. I HEARD 200 SAID 200; BUT BEING UNDER HEAVY WORKLOAD; AND JUST A LITTLE BIT SCARED MY BRAIN HEARD FL220 AND TUNED IT IN MY ALT ALERT. I KNOW YOU WANT TO KNOW HOW ATC CAN HELP. REMEMBER I SCREWED UP. THE ONLY THING THEY COULD DO IS WATCH CLOSELY IN SEVERE WX; MONITOR MORE CLOSELY AND GIVE US IDIOTS ALL THE HELP THEY CAN. THIS IS THE FIRST TIME IN 31 YRS I MISSED AN ALT.