There's a bewildering array of detail changes to the Sherman, depending on Manufacturere, Model and year. There's an 'early' low bustle, and late 'low' bustle for example , and they look distinct.
The High bustle was introduced to allow clearance for bigger hatches, as far as I can tell. Clearly, the Firefly needed a large loader's hatch (and hence the unique rectangular design we see on them); but the point I'm making- going back to the orginal theme is that it's:
1. Not necessary to have a three-piece gearbox to make a Firefly, nor:
2. an elongated M4A4 chassis, as the M4A3 hull will suffice. Similarly, High and Low bustle turrets can be used.
3. M4 ICs came with a mix of High and low bustle turrets, (some remanufactured from earlier models)- though the majority of Vc (M4A4) Fireflies had high bustle turrets. So, with a bit of skill the standard model turret can be modded to be a passable Firefly type without too much Sturm und Drang. There are examples of it in the archives here. One of the best (if not THE best) is here:
viewtopic.php?f=21&t=7885&hilit=M4A3+co ... to+Firefly The turret mods are outstanding, and relevant to a M4 1c build, although the hull is the M4A1 type.
The issue with converting a M4A3 model (105mm or 75mm) to a firefly is likely to be with making the correct size hatches, and having the correct glacis angle (60 deg).
here's a useful pic of the variations in glacis angle:
I think that all the later Tamiya nd HL variants probably have the 47 deg glacis. However, it's to a massive to modifcation for someone with a razor saw skillset.
Here's a quick develomental diag of the 75mm turrets Fireflies were derived from:
"Get your facts first, and then you can distort them as much as you please"- Mark Twain.