If the C10 does fit, it's probably going to have the same ratio. Could probably get a custom ratio box from somewhere, though I have no idea who would offer it. Maybe check out
Lee's Power Steering. My 72 Chevelle has amazing power steering and a quick ratio box. I think it is a 12:1 factory ratio IIRC. You could probably mount up one of those or a Vega box with very little fab work. I've also heard of guys using the Jeep Grand Cherokee boxes, but not sure what years.