[Coley]

To my recollection, the highest GVW rating you could get on our trucks was 5400lb rating on the GVWR plate.
As mentioned they upped this starting in 1973 and if you get a chance to look at the rear spring pack on a mid 70's heavy half/Big 10....it is very substantial....way more than the 67-72' group.
The other thing I believe the option called up was bigger (wider) brakes which were needed.
I've seen a lot of these 'heavy half's' carrying campers and when equipped with 16" tires...they seem to have no problems whatsoever carrying a pretty heavy load, particularly if you add on the Auxiliary/Overload spring system on top of that already substantial group of Heavy Half' springs.

Now, that said...it is not uncommon to see 67-72's (or even the next earliest series 64/65/66') running around with a designation on the front lower side of the box saying 6000 GVW...which I find interesting.
I have seen this hundreds of times on older trucks.....who made that call?... and wrote/stamped that writing on the side of the box?....it wasn't the owners.
Interesting stuff.
All Good
Coley