I offered up the 257mm on my current 14" rims, and in comparison to the standard 240mm there isn't a lot of difference. Infact I seem to remember reading a post where someone put the 257's behind 13" rims. I think you're type of caliper is not as tall as the standard one. I think it will fit.
I offered up the 280mm alfa brakes behind 14" rims, they were only 11mm out from fitting.
Do mtec do drilled and grooved discs in that size?