I think Wrenchbender and Oldsub have got it mostly right. The 4.57 gears aren't going to help you get 70 mph regardless of motor. Unfortunately you probably have an Eaton rear axle (removeable pumpkin), which means the best gears you will be able to find for it are 4.11. I'm a little surprise you have a 230 in a C20. A decent bent thing will cost a few $$$. I'd look for a 292 that someone has replaced with a bent thing. The 292 will have the bottom end of a 350. My 63 C20 with a 292 has hauled a cab over camper many miles. With out the camper I've have hauled more than few trucks on a trailer. One with a full size utility bed, I was down to 35 mph on a long grade, but the 292 will do that all day long.
Larry
