In operation, Bumper Jacks are dicey tools at best. The basic design concept is just really weak. Often, as the car is jacked up higher and higher and the emergency brake holds the wheels stationary, the load location shifts further away, relative to the jack's very narrow baseplate.
Sometimes they are used to raise a car so high that the jack shafts actually bend. Not a great whole lot, but enough to make a reasonable person leery. Every so often the news carries a story about one failing, resulting in injury/death.
Seems like you should obtain either a jack specifically made for the '71 Duster or, at minimum, a bumper jack with a very definite underhook that will closely match the Duster's bumper contour.
Maybe a trip to the auto boneyard or perhaps an ad on Craigslist will help you in this situation.
Good Luck.