subframe bolt nuts

[NO NAME]

On the inside of the inner sill at the front is a box section that has a nut in it (I think, my car had lost all of this with a previuos owner) I am putting in a box section attached to another box section that is replacing the front rail under the floor.
It should all work in a way that if you took away the sills and floors the car was still hold together.
I still plan to use original subframe bolts wich I havent yet got, tomorow I should be upto a stage of were it is required to weld these nuts into the box.
does anyone know what sort of nuts to use that I will be able to bolt the subframe bolts into?

[NO NAME]

I was just looking at hildas make over, I think I am picturing wrong how it must go.
anyone got a load of pics of the front sills going together.
I think I might can the idea of pressing out the raised bits on the inner sills were they meet the subframe and try sarges approach tommorow.

[NO NAME]

doesnt matter know I have gone a different way and standard bolts wont be usable.