Convert 67 to 68/ later tank straps?

Simple really. There is a cross member with the upper shock posts attached. The slots cut in this member are huge block letter like T shape. The tank straps drops right in those. The 67 will have one example of the T slot cut into its left rear sub member.

If someone wants to go under a 68 up and pull measurements, that's great but...
all you really need is the later tank and straps. They will point at where the new slots belong.
Drill three 1/2 inch holes in a T formation and clear out the remaining with dremel, file, whatever.
The tough part is the floor of the spare well. There are a couple of pieces that stack up and weld together to create the tank support and spare wheel anchor.
I have seen those at repop websites before but they were pricey.