I have purchased aftermarket panels from several places over the years- you CAN NOT expect perfect fit, as the car you need the panels for will not be perfectly straight if it's been driven ever.
A 1970 anything will have all kinds of particular quirks unique to it that can not ensure a perfect fit from any aftermarket part, be it fender, floor, sail panel, etc. Bodies flex over time, so the new panel will not have built in adjustments to the flex your particular car has received.
As mentioned, almost all sheet metal suppliers sell pieces made from the same manufacturers. Buying from one place vs. another should be about service, price, and comfort level. You're likely going to get the same part no matter where you order it from.