A DC8-55 FREIGHTER CAPT OVERSHOOTS HIS ASSIGNED ALT WHILE HAND FLYING THE ACFT.

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ACFT WAS BEING VECTORED FOR SEQUENCE INTO SEA; FL310. ATC GAVE US A HDG TO FLY AND A DSCNT CLRNC TO FL290; THEN CLRED US TO FL260. AUTOPLT IS A VERY OLD MODEL AND WAS VERY JERKY; SO I DECIDED TO HAND FLY THE ACFT OUT OF FL300. BOTH THE FO AND I HAD OUR INSTS SET FOR THE GLCR 3 STAR. INSTS ON THE FO'S SIDE WERE GOOD; MINE HAD A FLAG. THE POS DEV INDICATOR HAD BEEN WRITTEN UP SEVERAL TIMES IN PREVIOUS DAYS; BUT THE WRITE-UPS HAD BEEN CLRED. I HEARD THE ALT ENCODER GO OFF AND CALLED '1000 FT TO GO.' WHEN I LOOKED AT THE ALTIMETER; WE WERE DSNDING THROUGH FL255. GOT THE ACFT BACK TO FL260; JUST AS ATC CLRED US TO FL240. THE FLT WAS COMPLETED WITHOUT FURTHER INCIDENT. OBVIOUSLY; THE NAV INST INDICATIONS TOOK UP TOO MUCH OF MY ATTN. CONTRIBUTING TO THIS LOSS OF SITUATIONAL AWARENESS ON MY PART WERE THE NONSTANDARD PANEL LAYOUT AND INSTS IN THIS PARTICULAR ACT; AS WELL AS THE KNOWLEDGE OF PREVIOUS MALFUNCTIONS IN THE RECENT PAST. IN ADDITION; HAD I LEFT THE AUTOPLT ON FOR DSCNT; I WOULD HAVE HAD MORE TIME TO CHK ON THE ACFT'S FLT PATH; AS WELL AS THE INSTS. THIS OCCURRENCE SHOWS THE IMPORTANCE OF HAVING A STANDARDIZED FLEET WITH THE SAME LAYOUT AND TYPE OF INSTS ON THE PANEL. IN ADDITION; FLC SHOULD BE EXTRA ALERT WHEN FLYING A NONSTANDARD ACFT; AND MAKE ALLOWANCES FOR A SLOWER-THAN-NORMAL SCAN; DUE TO THE DIFFERENT COCKPIT LAYOUT.