I agree that you are not going to pin down a date, but there are going to be some specifics that can be said about the 3 "dimple" upper firewall on a '70 e-body.

Did LA use this part? With e-body production ending in May '70 - did they recieve any of these firewall panels?

Since it appears April in Hamtramck was the some do & some don't "month" are there any built in March with the "dimples" (& I'd say any in Feb. '70 or earlier would be suspect), or any in May without the "dimples" (later w/o would be suspect).