Your setup is exactly what I have been planning to build for myself. I hope this hasnt been asked already, but what changes would I have to make to your final drawings and parts list to use my 5500W ULD elements in both HLT and BK?
Is there any particular reason you want to use 5500W elements instead of 4500W? If you used my setup, but with 5500W elements, you'd be running awfully close to the 50 amp capacity of the GFCI. I would probably suggest using a higher capacity GFCI if you went that route. You might want to look into sources for a higher capacity GFCI breaker, and about how much it will cost you over the 50A GFCI breaker.
TB