Consider the forces that the seat frame has to cope with in the event of a collision. If you modify the seat base or the connection to the frame rails, you run the risk of catastrophic failure in a severe collision. I would not modify the frames; the risk of failure isn't worth it.

Instead, you could consider having custom cushions built for the OEM frame. A competent upholstery shop could fabricate a new seat cushion that is firmer and thinner, which would grant you about an inch of headroom by positioning the seat lower on the frame. That said, the lack of dampening may make the ride too harsh for long trips. :dunno:

Realistically, the most logical choice IMO would be to purchase a set of Del Sol seats and have them reupholstered to match the rest of the interior. Yes, the design of the seat is different, but you get the lower seating postion and preserve the dampening effect of an OEM seat cushion.