I have a starter motor which I have been told came off the crossflow/4 speed 'box I intend to fit.

It appears to be an inertia type but it only has a two bolt mount, whereas the 'box has three holes.

I wish to grind of the third hole/lug due to a clearance issue.

How can I tell that the starter I have is correct for engine/gearbox prior to fitting it all in car and trying it? Are some crossflow starters two hole mounts?

Tar.

my starter has 3 holes, but i'm also thinking about grinding one of for clearance.

My xflow has an inertia starter - three bolt.

Cpuldn't honestly say if there is a 2 bolt inertia. I believe there is a 3 bolt pre engaged version.
For what it's worth, my new / exchange one cost me 20 quid + vat at LSUK last week.

Why not bolt it to the engine and try it, all you need is a battery and a pair of jump leads.

afaik al pre x flow.s were 2 bolt,they kept falling out and were modded to be 3 bolt,so better to stick to the right 3 bolt item

Not all Escorts had 3 bolt starters BUT I can confirm the 2 bolts jam and chew ring gear .