[TowerTalk] suggested mast length for MA-770MDP?

[Howard W6HDG]

I know that I need a 2"OD mast for the MA770 that I have on 
order.  I'm trying to decide on the length of mast that I need to 
order.  The tower spec sheets are skimpy on how the round mast fits 
to the square top section of the tower.  Can someone please describe 
the attachment method?  In other words, if I purchase the 5 foot 
mast, can I fit 2 feet down into the square section or is there a 
hard stop that would prevent this?  I want about 3 feet sticking up 
for my Steppir DB18.

Thanks to the group for all the help.  Howard W6HDG