Looks like you are making what fiberine calls the "transit top"
Full Size Van Tops
I would think you would end up with about 76" interior with the 24" and a 66" with the 14" top, could you verify when you get yours on?
Do you have both tops at your place now?
IF I were to go this route I think an 18" would be the way to go, just enough for my 5'10" carcass to stand vertical.
That said, do you think the demand will be that great for a 5'6" interior height?
Thanks for offering this to the community.