68 C10 SWB speedo cable length?

[Chris78]

Hello fellow truckers,

I'm restoring my 68 StepSide C10, it's got a 454 paired up to a strengthened TH350 - let's see if it holds up, 12 bolt TrueTrac with 3.73 out back and 305 wide M/T tires under tubs is the plan.

It doesn't have a speedo cable, and I'm in need of one - none of the known vendors are able to tell me what length the cable should be however, they run from 80in all the way to 120in it seems, 4 different ones on option.

Any of you guys out there able to narrow it down for me?

Thanks,
Chris