For a long time, I built up GT6+ boxes from a recipe that involvess
a Mk.3 input gear, a new main shaft from John Kipping and a TR7
reverse gear; this works pretty well for my autocross use, though
I have had a tendency to go through synchros quickly (better
that than the small needle bearing on the nose of hte mainshaft,
which dies, spills needles into the box and chews up the nose
of the mainshaft - ugh). Kipping was selling similar boxes
for a very reasonable price; I haven't talked to them in while
but I suspect they still do.
I certainly wouldn't trust VB to build me a gearbox.
I don't think the Supra box will fit without hacking the frame
quite a bit. The smaller Toyota box can probably be made to
fit; there's a gent in New Zealand who makes a business of
this kind of swap - look for Conversion Components in
the monster list. I've talked to him about a GT6; he's never
done one but would like to, and I haven't yet had the need.
The Factory ran TR4/TR6 gearboxes behind the LeMans Spits as
well as GT6s; this is an easy swap, but you won't be able
to fit the length of an overdrive version between the frame rails.
I've corresponded with one fellow on the net who was pushing a TR8
5 speed into a GT6; he said it didn't take a lot of frame hacking.
About diffs: the weakest point of the GT6 diff is the spider gears. My
solution was to get rid of them by installing a Quaife, which gets
me a LSD at the same time, and doesn't require a lot of
fabrication. The "period" upgrade to the rear end was to
put a 240Z diff and half shafts into the back of a GT6+;
this is how Group 44 built their championship winning cars in
'69 and '70. The conversios I've looked at involved a lotof pretty
welding; if my other projects get out of the way this winter,
I'm thinking of making up some custom half shafts and an
adapter plate to replace the rotoflex halfshafts on my GT6+
but it will be a bolt-up conversion.
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