My 1970 had the chrome distributor cover. I know by 1978 the HEI was being used as a distributor and the chrome cover dispensed. So somewhere in between it was discontinued. I am not sure when.
If your car had the chrome cover, you could fit that over the distributor with brackets attached and see where it lines up. It's a pretty close fit so where the mounting brackets bolt up should be readily apparent.
If not, I don't know for sure.
It's possible that they were staggered, the rear bolt on the drivers side and the second bolt on the passenger side. This configuration puts them approximately straight across from each other.