I could imagine a cast hull for a Churchill being assembled from several pieces, bolted and/or welded together. Bow, centre, rear of the central hull are separate and the side sponsons as a single piece, to which the suspension was attached, above and below. That would in turn be covered in pieces of mild, rolled steel to prevent ingestion of vegetation and earth. The British were, by war's end making extensive use of combined cast and rolled armour elements (turret walls were cast and the turret roof welded on). I don't see why that couldn't have been arrived at earlier, with perhaps a central section of the hull roof, where the turret ring was being rolled and welded on. This would speed and ease production considerably.
As to choice of gun, it would more than likely start with 2 Pdr an a mix of 3in How. Quickly replaced down under with 25 Pdr as the emphasis changed from AT to infantry support. I agree that the Mk.IV would be more than likely the best shape to start with. 75mm was an imported calibre, I'm not aware of any 75mm production facilities but I'll check the official history this evening to make sure.