CCV Ford E-Series (thxs yelnam) tops per my personal measurements: (corrected info from measurements taken with a tape vs my memory
High top - outside: 19" measured from the top edge of the gutter. About 5'8" clear room inside with the top down, lots of usable storage a was with top down. Under bed has a bit more head room than stock roof.
Medium top - outside: 12.5" measured from top of gutter. About 5' 0" clear inside. Able to store sleeping bag/pillow on bed, storage above cab is limited and not really accessible with the top down. Under bed has a bit more head room than stock roof.
Low top about 5.5" to 6" above gutter, bit lower than SMB by about 1/2 to 1" - very limited production, and as Matt said only if you insist. The lift mechanism has to be lower inside, as does the bed support panels. The head room under the panels is a bit less than stock by about an inch to inch and half.
I might be off a bit in interior measurements but not by very much. I can get exact measurements in the morning if needed.