Not sure where to find out so...

I assume a Ford MT75 (5 speed) gearbox will NOT fit a 1600 Pinto, is that correct? Any way of using it in a Locost, or shouldn't I bother?

it will fit the engine, it may be tight to get it in the car.

locost not exact to the book

The MT75 has an even lower first gear ratio than the type 9 so you wont be using first effectively giving you a four speed box.

the side of the box protrudes into the pedal box area as well