The Pre-63 Darts were B bodies (and heavier). For the most part, more rigid safety standards made the A-body Darts heavier over time. Door side crash supports (68 or 69), shock absorbing bumpers (73 & up), larger tires, wheels and brakes, etc.

Your 68 Post is probably one of the lighter cars, 1/4 mile racers tend to like them (although they may have been lighter because the Posts usually had fewer options than the Hardtops). Your Post seems to have been better equipped than most, guessing by the optional stainless trim around the side windows (probably part of an option package??).