A bit off the track but I had a similar problem with my Dakine seat harness of a similar erra. I tried on modern seat harness's but couldn't find any suitable for a woman.Some had high backs which don't fit our curvier back..

and hit my upper back as a result. The others had a clip system that was too hard to open with a females little fingers.
I couldn't get my old one tight enough as I'd lost weight.Luckily a trip to the sailmaker for some restitching + a piece of foam padding in the back has given it another lease of life.