Am I right in thinking the 1.25, 1.4, 1.6, 1.7 SE motor bolts up to an MX5 gearbox? Same as the big Duratec?

No, duratec and sigma differ

You need a Bellhousing to mate to a type 9

I have one supplied by BJP Services, along with a clutch and clutch release arm (And a lovely lightweight TTV Billet steel flywheel to match)

Or TTV now have an adaptorplate and flywheel available to mate to an RX8 6 speed I believe.

I`m currently planning another adaptor plate solution that will give you a close ratio 6speed on a Sigma. (When I get a chance to measure it all up)
I have a Box, I have a spare block, what I lack is time

[Edited on 27/2/13 by cr500dom]

You could save yourself the hassle and use the caterham box