I'm 6'2" with a '28 Coupe and have the same problem; the cheapy suction-cup prism from Bratton is a big help, but I would like to try one of those nice "Guide" prisms form the '50s...
Moving/lowering the seat in a coupe involves cutting down the seat riser and / or the backrest mountings; it's not easy.
You might try removing the seat bench, and replacing it with a couple boards and phone books, to see if/how much lowering the bench would help... before you get into extensive mods...