As it turns out the 400 version black 4x5 mixing box does fit the 500 hL. As it did in the 400 head it drops down into the bottom of the head a bit but the holes in the sides line up with the bulbs perfectly. The holes are oval shaped compared the the 500 version which are square but makes no difference. I bought the 500 head without any mixing boxes so I was hoping my 400 box would work and it did.
Like most other people I quickly found out the exposure times with this head are way too short with two 300 w bulbs so I searched online for a good week looking for just the right 150 w bulbs the slow it down a bit. With those bulbs most times are in the 20sec range. Just right for me.