I had one. Slow doesn't even begin to describe it. That added to the 6-8 mpg in town was enough for me to hate the car. I'm sure there was something fundamental wrong with it, but it ran good until the last few weeks that I owned it so who knows.
A guy I went to high school with had a 67 Skylark with a 340 in it. The windshield got busted out by a falling tree limb and he couldn't afford to get a new one, so he wore grinding goggles to drive for a few months! Funny, the crap that comes out of the memory banks when a trigger pops up!