replacing speedometer cable

[scarborough]

have a 1987 old's cutlass in need of a new speedometer cable don't have original cable. does anyone have a part number are know the length of the original cable. the car has a th350 mount to old's 455 engine. also i have a pair 255/60/15 that's going on the rear is their any changes i need to make to the speed gear to accommodate that set up

 

[fleming442]

Nominal height for the G-bodies is 26" which is 442 spec on a 215/65-15, however your 255/60's look to be an inch bigger at 27.
Play with this: https://www.tciauto.com/speedometer-gear-calculator