Based on what I'm seeing, it seems that a 68 L78 car (which is what the subject car of this thread claims to be), should have the fuel line mounted to the cross member on the passenger's side by a metal strap held in place by one of the motor mount bolts. Am I correct?
Yes and No.
You can not hang your hat on just that clip. To help answer a little further.
I feel the clip on the frame mount was used on all 67 & 68 big block cars. One thing that concerns me is the fact the fuel line was cut in that area as well as I do not see a clip or bolt or even a hole for said items on the top of the frame that secures the line. So yes this could be a big block car. Unfortunately, that is not enough evidence to sufficiently say that it is 100% without a doubt a big block car. What you need with any of these cars anymore is documented ownership history, original drivetrain and/or original paperwork. Of course the best is all three but sometimes 1 or 2 out of the 3 is enough when all other items check out and nothing seems shady or fishy. Hope that helps.