I recently ran into the same issues with straps. You can not buy just the straps and get it to work.
The ones on my 85 were riveted onto my support brackets.
The brackets were so rusty, they needed replaced anyways.
When I called truck they did not offer the support brackets. I got the brackets and straps already attached for $150 for the pair at the local Chevy dealer. It took the parts guy a while to find it on the diagram. I think they call it the "gas tank support bracket".
Hope that helps.
David