Check the fender tag. Rallye cars will have the A57 code on the fender tag (and build sheet if it still exists).
Not all 340's were rallyes and not all rallyes were 340's (or in 74, 360's). My 74 rallye was a 318 car.
Rallye cars will have the bulge hood, rallye gauges, front and rear sway bars, and the fender moldings behind the front wheel opening.