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Not sure how this will relate to your '78, but I measured the staps in my '71 coupe and came up with this data:
From the "hook end" of the strap that connects to the removalbe lower support bar, there's about 1" running horizontally forward, then 12" going vertically up in front of the the tank, then 13" running back horizontally across the tank top and finally 8" or so going down vertically along the back side of the tank to the rear frame crossmember.
Just don't make your pre-bends to tight so they'll be able to "pull out/ease/stretch" to comform to your actual tank shape.