Remember when you're dealing with turrets that earlier types had the 'low bustle' turret. You're very unlikely to get the early mantlet on a 'high bustle' turret with leader's hatch. When it comes to the Firefly, most were converted using the 'low bustle' turret.
As UVS mentioned, most 105mm turrets had a second ventilator on the bustle. They'd also normally have the later cupola rather than the split hatch. Something to keep in mind when it comes to designing the actual kits.
If I was designing the Sherman range, it would be something like this:
M4 - both mid-production welded hull and late (composite) hull, 'low bustle' 75mm turret (with narrow and later mantlet) and optional parts for a Firefly. This would cover the Commonwealth and US Army.
M4A2 - both mid and late production hulls, 'low bustle' 75mm turret and early 76mm turret. This would cover the Commonwealth, Red Army and US Marines
M4A3 - late production hull, optional tracks with extended end connectors, late 75mm turret and late 76mm turret, optional parts for 105mm turret (i.e. basically the same as the discontinued kit, which had an excellent choice of variants). This would be exclusively US Army
For interchangeability, the engine decks would be separate, so that (for instance) the late M4A3 hull can be used with the M4 to make an M4 (75mm) - which would leave enough parts for a mid-production M4A3.
