I installed a Wittner tailpiece with integral fine-tuners on my Holstein Red Mendelssohn fiddle today.  Here is how it went.

The Wittner is about 5mm shorter than the tailpiece it replaced so the bridge needed to be moved back about 4mm to maintain the proper 1:6 ratio between the after length and the sounding portion of the string.  The soundpost was also moved along with the bridge.  

Attention must be paid to the ball-end of the strings during installation in the fine-tuners to make sure they seat properly in the slots.  You either need to make a bend in the string or use some sort of tool to guide it in the slot.  Either way you will need to look underneath the tailpiece to make sure they are located properly before putting final tension on the strings.  The fine-tuners work smoothly but don't have much travel so you will need to tune the G, D and A strings within a few cents with the tuning keys.

I didn't know about the difference in length between the tailpieces before I started this.  Wittner also has an "Ultra" version of this tailpiece which has a 5mm adjustment of the tail-gut length which can be accessed after the tailpiece has been mounted.  That would allow me to move the bridge back to the original position.  I will probably eventually opt for that one although I don't notice that this "upgrade" has harmed the tone of my fiddle at all.  That is just me being picky.