Ok. Just acquired a '75 K20 Custom deluxe 2 wheel drive. She's got the original 292 i6 with 4 speed manual. Fired her up to bring her home. Starts good. Idles good. 1st and 2nd can run through just fine. In 3rd gear and more so in 4th gear when I get her up to speed, around 40 to 45, it's as if...