Gearbox Doesn't fit

Likely the wrong spigot bearing.
Look up the size of the spigot bearing for your engine.
18mm might mean it is a gearbox for a bigger engined car. That is big.
No way will that bolt up.

Edit: from what I could find the internal diameter of the spigot bearing is 15mm. 
if the input spigot shaft is 18mm it must be the wrong box.
Maybe from the V6 which would have a bigger input shaft. That's a guess.

The end of the spigot is flush with the gearbox mounting face. The boss on the end of the crank which protrudes through the flywheel with the 13mm hole in it is 5mm further out from the engine mounting face.

That may be the wrong bearing. Actually it's a sintered bronze bush.
No way should it be proud of the flywheel. They are normally recessed so they play no part in anything other than locating the gearbox shaft.
Let me do some more digging.

If you have inherited it like this then someone has fit the wrong bush when they changed the clutch..that's not normally done or they may have had a noisy gearbox and bought the wrong one. There isn't another answer really.

More digging...

According to what I can find

..."the PG1 gearbox fitted to the 135 does NOT have a crankshaft spigot bearing.
It relies on the front input shaft bearing of the gearbox to stabilise itself"

If that is so then it is a shit design as far as I am concerned!
I think a quick call to one of the MG specialists like Rimmers or MGFnMGTfbitz would confirm that very quickly.

The following pic is taken directly from the MG TF Workshop manual.
It lists all components and the diagram of the components.

 

The PG1 is a Honda design & arrived with the Rover 800. Goodness knows how many millions have been made & they’re all basically the same, ratios differ & the odd ones have a Quaife limited slip diff but other than that, fairly bogo. Bogo & reliable.
I would have thought it would be better to harmonise the engine fittings to accommodate any PG1 rather than the other way about,
M

In that case remove the spigot bush.
Somebody must have fitted one incorrectly.