I don't have a tumbler big enough, in fact not many do.
That would be best answered by 68 Shelby owners.
Check the 'Services Offered' section... there's a guy in Apex, NC that offers tumbling services
My '68 Shelby didn't have the staggered shackle, but I wouldn't focus on Shelby cars only as it was a dual exhaust detail that would have been done at the Ford assembly plant.
Tim has been into more '68 cars than most, if not all, of us, so his opinion, along with everyone else's is sound enough proof for me.