If you're really desperate for a 6-speed box, i know Quaife sometimes have gearboxes available that are direct replacement for the factory unit - but have different internals. I had a UKDM Ford Type-9 Style 5-speed in my Sierra XR4x4, and after the engine swap (2.8litre 160bhp OHV V6 came out, 550bhp 3.0 litre 24v QOHC V6 went in), the box was the next thing on my list. Quaife did, and to my knowledge still does, a 6-speed version of the box, made with all alloy main case, alloy belhousing & quaife's own transfer box tailhousing. Cost the better part of ?6000 (about $9500?) but damn it's a good box. Virtually indestructible!
I suppose all i'm trying to say, is check out Quaife :P