Bob, if the 5th gear feels very high, the odds are its the earlier Ford MT75 gearbox with the 0.76:1 ratio. The MX5 box I believe has the 0.814 ratio.
When I was in the factory late last year there were still a couple of 4/4s with the Ford gearbox and I believe that the Plus 4 was fitted with them first as stocks of the older boxes were run down.
