Probably well documented on here, What bits are needed to put a Mazda 6 speed onto a ST170 engine

Get an rx8 box, 6 speed same as Mazda mx5 mk3 but has a Ford input shaft spline so can use ford stock clutch plates (mx5 different as us he same box in the is200, 200sx, brz and gt86, all aisin az6). Freddy686868 sells adaptor plates on eBay or via turbosport (lots of info on there in the trechnical>running gear section) then need to decide to go hydraulic clutch or convert to cable plus may need to elongate the clutch arm pivot to get the right arm position or space the release bearing, depends on clutch you use.

Also note the 6 speed box has a very low 1st gear (3.75 iirc) so in most cars this box becomes a helical close ratio 5 speed (just use 2-6) with a reasonable cruising top gear (0.85) that is able to easily handle 250hp/250 Ftlb, very comparable to bgh e8, quaife clubman hd, spc duratec semihelical but at 1/10 the price and less noise.

Note if you did not know st170 is a zetec not a duratec it is the later all alloy duratec that fits the Mazda box (but canted over slightly). St170 has the original bell housing bolt pattern, same a xflow, pinto, zetec, but duratec very different (as us zetec se).

Hope this helps.

[Edited on 18/3/17 by Ugg10]