I do not think that the 511 top decks fit the 501X. 511 has new bulkheads.
Take a look at this:
http://www.tamiyaclub.com/showroom_m...87535&id=13377
By the way, I ran my 511 on a bumpy 1:8 track with a lot of big jumps. I think the new suspension arms (high traction ones) can take a lot of abuse. I was impressed.