To update this thread, my '08 QP took an H8 battery, which is significantly longer than the H6 noted in the cars above. I also saved the battery vent from my factory battery and it plugged right in.
Either MT-49H8 or MTP-49H8 will work, just have differences in warranty basically.
'08 Quattroporte (Daily Driver)
'05 Viper (Mamba Edition, TT, 1,400whp)
'98 M3 (Race Car)
'03 X5 (Tow Vehicle)
'99 Eclipse GST (I don't know why I still have this)