Gearbox crossmember removed, bad corrosion!

Hello

I noticed my bolt in cross member was very rusty and flaking apart so removed it with the intention to weld in a new strut. Once out, behind where it mounts is very rusty and flaking apart also on the chassis rails. How can this be repaired without a new chassis? Was thinking weld in plate which I can get to at the sides and bottom but can’t really get to the top. And then make the cross member a couple of milimeters narrower to compensate for the plate? Or could I just cut it out?

cut it out , you can remove floor plates to get access to the top

I greased that area on mine years ago,changed the crossmember last year and was still good.