Sometimes it can be hard to work out the ID, like old holdens, parts swap easily.
I think up to 71 (maybe 70) the front down tube bends underneath the engine where the outer frame rails meet up with it.
After 71 the front down tube finishes shorter & is straight & the outer frame rails bend up on an angle to meet up with it.
Up to 73 the exhaust fitting on the barrel was a bolt on with flange.
74 exhaust was a slip mount with springs.
75 exhaust was a slip mount with springs & a very short hanger bracket for exhaust mount.
I have come a cross a few that have had barrels changed.