With properly applied, correct high temp glue and a heat gun used with judicious skill and care, virtually any vinyl is heat-form able. I use to cover car dashboards in my upholstery shop, most with no seams. I don't know how the Bolens seat came, whether it was 1 pc or 2 pc foam, but often it's easier to fit the foam in 2 pieces, bottom and back, or two layers of thinner foam, glued carefully to the pan.
I'm going to step back, because 29 Chev doesn't need my help, he does beautiful work! I just thought someone might get a bit of help from my experience.