You will likely have to go with nylon MTB straps. Vintage isn't always worth it if it isn't functional or comfortable. If you have a clipless system on your other bikes a clipless system will make this ride more enjoyable.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk