Actually, it's a pretty easy solution: You need a 66/67 speedometer cable which has the large nut for a 66-up pinion adapter and the correct smaller end for your speedometer. You need the pinion adapter and the appropriate pinion gear for your tire size and rear end ratio. I've got the outer cable (but no inner one), pinion adapter and various size gears that I'd swap you for your 65 cable and gear. I have a 65 inner cable and can check to see if it's the same size as the 66/67 A Body outer cable
