Lots of great advice as always. It's useful to know the original was Jute/hessian backed rubber butyl.
You are right about the 3M product, it is thin - about 1/16". It can be doubled up (as specified by 3M) to make a layer the same thickness as the original. However, it doesn't look the same and I'm not comfortable with it, which is why I put up this query.
Plenty to be researching here, including making my own, as you have outlined and checking out the companies you specify.
Many thanks for the company names and product references.
1974 Silver Shadow