From what I understand of the six speeds:
If rear axle gear is high numerically it makes 1st and possibly 2nd not very usable.
If rear axle gear is low numerically it makes 6th not very usable (on highway - might be different on track)
Reconfiguring the tunnel and cross member is quite a bit of work.
If one of the gears, 1st or 6th, are some what unusable you essentially have a 5 speed anyway, whats the sense in going through all that installation hassle?
If I remember right Jamie said he felt his trans would hold up to 750 ft lbs.
I know which way I'd go.