Rog,
I have a 6 litre Chev and a 6 speed in a Harrison. It came with 3.9 nissan rear end.
The engine was standard (they are pretty good by the way) but I didn't like the amount of gear changes I had to make. The cars dont weigh that much so they dont need alot of revs to get them moving.
I have a Cobra with a 5 speed and 3.55 gears, this has a great spread of power from 1200 RPM to 6000 RPM which is perfect for the odd sprint and will easily run at 110km/h at around 2100 RPM.
Therefore after driving the car for a while I decided to change the diff to a 3.54. It was the best thing I have done to the car ( besides the heads and cam that is
) it has become a much more diveable car, it has reduced the amount of gear changes dramatically and the car still has a heap of acceleration. Dont forget they only weigh about 1200KG'S. Alot of people that have not driven a Cobra dont quite understand the weight thing (I didn't ) they spend all their time on the web discussing this and that, researching stuff and disregarding the good stuff they are told.
IMO you should go with the 6 speed as it will give you the latest gear box and more final drive ration to utalise. See if you can drive one before you start to change things.
My 2 cents worth,,,,
STIFFY