If you are set on chopping your coupe, I would chop the top first and then fill the roof. May not make too much difference in a model A, as its mostly straight pillars etc. but if by chance you did have to lengthen the top it would be very difficult with a filled roof. I think all highboys should be chopped, but that's just me, however I'm not thrilled with the looks of filled roofs.

Good luck

Mick