BE-9L PLT HAD AN ALT OVERSHOT; AND LESS THAN LEGAL SEPARATION DEPARTING FSD.

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I WAS CLRED TO RAPID CITY AS FILED; CLB TO 5000 FT EXPECT FL180 IN 10 MINS. AFTER TAXIING TO RWY 21; I WAS CLRED FOR TKOF; TURN RIGHT TO 270 DEGS. I DEPARTED RWY 21 AND REACHING 1800 MSL I TURNED R TO HDG 270 DEGS; CONTACTED DEP AND TOLD THEM I WAS CLBING THROUGH 2300 FT FOR 5000 FT; HDG 270 DEGS ASSIGNED. I WAS INFORMED BY DEP OF MOONEY TFC AT MY 10 O'CLOCK POS; 5 MI AT 6000 FT. THE MOONEY WAS INFORMED OF TFC AT 1 O'CLOCK POS 5 MI; A KING AIR; CLBING. AT THIS TIME I WAS CONCENTRATING ON LOOKING FOR THE TFC (CLB RATE 1000 FPM). A SHORT TIME LATER DEP CTL TOLD THE MOONEY HIS TFC WAS AT 12 O'CLOCK POS AND 1 MILE. THE MOONEY CALLED THE TFC IN SIGHT AND THAT HE WAS PAST. I LOOKED BEHIND ME AT ABOUT MY 7 O'CLOCK POS AND SAW THE ACFT BEHIND ME AND ABOVE ME. I DID NOT CALL THE TFC IN SIGHT. I CONTINUED TO CLB. SOMETIME LATER; I GLANCED AT MY ALTIMETER AND REALIZED I WAS AT 6000 FT. I REDUCED PWR TO BEGIN A DSCNT BACK TO 5000 FT. AT THAT TIME DEP CALLED AND ASKED MY ALT. I INFORMED HIM I WAS AT 6000 FT CORRECTING. DEP REMINDED ME I HAD BEEN CLRED TO 5000 FT; CONTINUE CLB; CLRED TO FL180. FACTORS CONTRIBUTING: I LEFT THE AUTOPLT OFF TO BE ABLE TO MAKE A QUICK TURN IF NEEDED. I WAS CONCENTRATING MORE ON FINDING THE TFC THAN CLBING TO AN ASSIGNED ALT. AFTER THE TFC HAD PASSED BEHIND ME; I JUST CONTINUED TO CLB AND FORGOT ABOUT THE ASSIGNED ALT.