I know this is going to sound strange, but I look at a curved panel as sort of "bent flat panel". I block with flat tools and take an inch or so at a time. I have found that the rubber flexable blocks and things go in and out of the high and low spots, never really "flattening" them out.

1948 Chevy Pickup
Chopped and sectioned
owned since 1974 when I was 15.