Fractionally over 53 inches. It's a Mk V dichromat head, which is a bit shallower than a Mk III. My Varicon would need yet more clearance.
Apologies. Re-read, and realised you asked for height from the bench - I gave you from the base board, so you'll have to add another 5 inches. As for difference between 5108 and 504 heads, your guess is as good as mine.