318 Flex plate question/ help

Yup. Car vs truck isn't what matters. Up until 95 the weights for those engines was on the torque converter, not the flex plate.
In the 70s and 80s the only difference in flex plates was that there were 2 different bolt spacings, I believe it was between 904 vs 727, but there were 2 different size converters on 727s. 11" with wide starter ring gear and 12" converter with narrow ring gear. Hopefully that 727 has the smaller converter for your use in a car most 727s behind big block engines had the bigger converters, you should be good.