I replaced the factory moulds on our 2 door RS2000 with recently purchased plastic items.
Had exactly the same problem.
I used a hair dryer to help very carefully bend the strip.
Even before bending the mould was already way too wide and very loosely fitted the gutter, however when bent the width increased heaps, and the sectional profile and shape distorted. I had to apply fudge factor and use silastic to help keep the moulds in place otherwise they would easily blow off and possibly cause damage to the car itself.
If anyone ever comes up with a better idea I will happily replace them. Should have a time machine. I recall the Ford items as new replacement were quite cheap in the day.