I also had the 0.642 5th installed and it cost $1500 extra while the box was apart for a rebuild. Without a doubt one of the best mods I have done. There is a guy in the UK that has a good reputation with ZF's, although his name slips my mind (Phil?). Johnny Woods would probably know.
A bit of thread drift but I would refute Jack's (Bosswrench) statement that a 6 speed ZF is cheaper than a Ricardo, as reconditioned Ricardo's sell for $6500 and a ZF 6 speed is somewhere around $16K now. However the Pantera chassis requires heavy modification to fit the much larger Ricardo.
Another alternative is the ZFQ made by Quaife for the GT40 guys, it is dimensionally the same as the ZF -2, has CV outputs and a 3.75 diff as std. and cost was GBP6500.
Julian
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