Look for "evidence" on the front header and/or rearmost top-bow. You should see some signs of where they were located by faded/discolored paint or fabric.
On the two-seater (BN6, in my case) the rearmost top-bow has retaining clamps to secure the straps. Positioning them on the front (wooden) header was a breeze.
Randy
57 Healey 100/6 BN6L-942
61 McCulloch R1 go kart
99 M Roadster (supercharged)
01 M Roadster
11 BMW X5 35i