When to shift?

Well, I'm not really a /6 guy. Looking at the stock specs a '68 170 made 115 hp and 155 ft/lbs of torque, where a '70 198 made 125 hp and 180 ft/lbs.

So, that's not a big difference. The hp difference isn't something you'd probably notice by the seat of you pants. The torque difference is though. It's not like it would turn the car into a rocket ship. But as they say, there's no replacement for displacement. And anything you did to a 170 to build power would make more power with a 198. It's a bigger displacement change than going from a 318 to a 340, or a 340 to a 360, and we all know how people feel about those engines.