I just put my heads back on over the summer, I seem to recall the same concerns. I "think" there were a couple of small differences, been a while
and I don't recall having any big issues.
Try this:
-The housing for the distributor may only fit on one head.
-The hole where the distributor goes thru the head may only be
-large enough for the distributor to fit on one head.
-The back of the passenger head should have a small threaded bolt hole
-as a support for the bypass tube that goes under the intake.
-There may also be a corresponding mark on the block that matches the
-head. I think one side had a matching "P" stamped into the block and head.