But I wasn't talking about the (AFAIK very similar, but not identical to the NC) RX8 box though.

Thanks for that. I see that the adaptor plate is for a 6 speed box and the box is a 5 speed. I had had a trawl of eBay and found several very reasonably priced 5 speed boxes but that 6 speed boxes were about the same price as a type 9. I haven't managed to find out if the 5 and 6 speed boxes are the same so would take the same plate.

Lots to think about. I need to get the chassis started first before buying anything but obviously need to make decisions on running gear which dictate certain parts of the chassis.

If converting to an rx8 6 speed costs an extra £100 over a type 9 then surely it's worth the extra?

The type 9, and I've owned a few is very poor and much weaker when compared the Mazda box. The shift is long and notchy and doesn't have the nice short slick feel of a modern gearbox. The type 9 may be a more straightforward fit but not by much.

The 231 6 speed will fit on a 192 Renesis engine; you'll have to look at the clutch/flywheel assembly to see what, if anything, you'll need to change there -http://www.rx8ownersclub.co.uk/forum/viewtopic.php?t=65599

If the RX8 box has anything like the shift quality of the NC MX5 box, then it is well worth it, the NC is a beautiful box to use.

Also, all type 9's are very old now with a corresponding amount of wear.