One could go either the low Stug style:
or the higher Marder style:
The reason the Germans could do the Stug style is that on the Panzer III, the vertical plate in front of the driver is a separate piece from the bow and glacis plates, which meant it could be replaced by a modified one on the Stug with the cut-out for the gun. The driver's position was already offset to the left.
On the Matilda, the entire bow, including the vertical area around the driver and the box, are a single casting, which means you can't just replace the top half with a new piece. It'd haved to be the whole casting or nothing, and if you're doing that, you might as well design a whole new vehicle. Furthermore, the Matilda driver sits dead centre without enough room for a gun to either side of him, and although it looks like the hull is full-width, in fact, the hull proper is a narrow 'canoe' like the Matilda I (it was actually inspired by the Christie tanks) with triangular stowage bins to either side. This means that you can't just move the Matilda driver to one side without, again, re-building the entire front end of the tank with new castings.
The Marder style might be doable, but again, you have the problem of the central driver's hatch, which means the new gun barrel can't really be mounted any lower than the original turret's 2 pounder. The Germans did actually try this with some Matildas they captured in France, the result being the 'Infanterie-Panzerkampfwagen Mk II (e) mit 5 cm KwK L/42 ("Oswald")'. As you can see, the Oswald was not exactly an impressive fighting vehicle:
(from here:
http://en.valka.cz/topic/view/12466)
It might be possible to do an allied version of the Oswald with a 6 pdr, or more likely, a 75mm howitzer to provide HE support to the infantry and gun tanks.