Auto Racing: REAR END, spline axles, bolt posi

Question
JOHN,  I'M BUILDING A STREET 69 EL CAMINO.  TRUSTING A LOCAL FRIEND, I PURCHASED AND INSTALLED A 10 BOLT (POSI 3.73) FROM A 85 MONTE CARLO, REPLACING THE STOCK 12 BOLT.  I BOLTED THE NEW REAR END UP, USING THE JEGS CONTROL ARMS FOR THE 68-72 A BODY.  THE CAR IS STILL UP ON THE STANDS, AS I AM STILL FINISHING THE BODY, WITH THE REAR END INSTALLED BUT NOT YET TESTED.  I HAVE MY DOUBTS AS TO IF THE REAR END WILL WORK.  ANY INPUT WOULD BE APPRECIATED.  THANKS IN ADVANCE.   SCOTT

Answer
HI SCOTT,I DO QUITE A BIT OF THIS TYPE OF WORK AND THERE ARE 2 PROBLEMS I CAN SEE .THE 85 MONTE REAR IS NARROWER THAN YOU 12 BOLT (54" AS COMPARED TO 51" ,BACKING PLATE TO BACKING PLATE)YOUR WHEEL/TIRE COMBO MAY HIT THE FRAME OR INNER FENDER WELL.THE BIGGER PROBLEM IS THE UPPER JEG'S BARS I BELIEVE WILL BE TOO SHORT THE 10 BOLT HAS IT'S UPPER MOUNTS ABOUT AN INCH FARTHER BACK THAN THE 12 BOLT ,THAT'S WAY TOO STEEP A PINION ANGLE ..,WITH THE CAR ON THE GROUND CHECK THE PINION ANGLE.(DRIVESHAFT ANGLE TO PINON/YOKE ANGLE ,NOT TO THE GROUND )1 OTHER GLITCH IS THE MONTE MOUNTS THE COIL SPRING CENTERED ON THE TUBE ,THE 12 BOLT HAS A FORWARD MOUNTINGON THE AXLE TUBE ,THE COIL WILL BE ON A STRANGE ANGLE AND WON'T COMPRESS CORECTLY ..NOW THE BIG QUESTION ...WHY ON GOD'S GREEN EARTH WOULD ANYONE TAKE A 12 BOLT THAT 1.ADDS TO THE VEHICLES' VALUE AND REPLACE IT WITH A PIMSY,WEAKASS ,INFERIOR REAR END WHOSE ORIGINAL POSI/RING AND PINION HAD THEIR ORIGIN  IN 4 CYL. VEGAS? A SWAP I DO VERY FREQUENTLY IS A 68-72 12 BOLT INTO MONTES AND MALIBUS ,WITH THE EXTRA WIDTH AFTERMARKET WHEELS FIT MUCH BETTER(15X8X4.5 BS FIT PERFECT).I'VE NEVER HAD A CUSTOMER DO THE OPPOSITE UNLESS IT WERE AN EMERGENCY.I HOPE THE MONTE REAR WAS FREE.I'M  GUESSING YOU HAD YOUR OWN DOUBTS ABOUT THE 10 BOLT(ONLY 27 SPLINE AXLES AND 2 1/2" AXLE TUBES!)I HOPE I WAS OF SOME HELP ON THIS MATTER .GOOD LUCK, JOHN MAZIO