[russellw]

The MTX75 does require quite substantial modifications to fit it into the Fiesta and I suspect you'd struggle to get compliance here.

The IB5 isn't a bad gearbox but it's designed for reasonably light duty work and is known for wearing out the synchros if subjected to punishment. Replacing those will probably fix both symptoms you have now and might be all that is required.

The B6 is a stronger unit as fitted to the later ST models and apparently is a reasonably simple swap but they might be hard to find here.

There are also kits with stronger internals (like the ERST ones) that include bigger bearings and better synchro rings but you'll probably need to source them out of the UK.

Unless you plan to race it, give the LSD a miss.