OK, that is clearer. However, while you now don't want to change US Army military theory, you need to change Industrial attitudes, which IMO would be nearly as difficult. US industry would want to use as much US componentry as possible because it would be quicker and easier than trying to manufacture anew British componentry. The Merlin is a good example of that. It might have looked like a Rolls Royce Merlin and it might have worked like a Rolls Merlin but under the skin, it used US fasteners and components. They were arranged in the same way but they were sufficiently different to the original Merlin that a completely new series of marks were assigned to them to differentiate them for the mechanics working on them.
I wasnt aware that the Churchill's turret ring was "nearly" the same diameter as the Mathilda. You learn something new everyday. While it might have made it easier to mount the Churchill turret, I somehow think that US industry would look rather askew at the Churchill turret. No doubt they'd like to mount their own design of turret. As for the weapon carried, the British were enamoured for quite a long time of mounting dual weapons - the 2 Pdr and the 3 inch How. One for AP the other for HE/Smoke. I could see the US manufacturers scratching their heads at that and preferring the 75mm. Even so, the ring would be too small to fight from. They would want a larger one for crew comfort and fightability.
They would sneer at the 'tilly's suspension. They'd rather use the US standard VVS suspension. So, ultimately, you'd end up with as I've said, a lower, wider, faster Matilda with a more powerful engine, a bigger turret and a bigger gun. It would take about 2 years to design and develop to manufacturing standard. It would be a good tank but it would not look like a Matilda.
As for manufacturing downunder, we didn't really have the manpower to sustain aircraft, ship building and tank building. Particularly by 1942 which is when I'd expect this to come on line. It was one of the major reasons why the Sentinel was dropped. We redeployed the manpower to ship building, which was used in the offensive against Japan.