In fact, you don't need to call it an M4A4 VC Firefly, you could simply call it a Firefly 1C. The M4A4 was made longer than all other versions, in order to house the bulky Chrysler Multibank engine. Some daring Sherman fans here have sawn the hull, and stitched back together in an elongated form to echo the M4A4 dimensions. That's a lot of tricky work.However, by altering the glacis angle on the M4A3 (105mm) from 47deg to 57deg, as you've already done, you have the makings of the M4 1C Firefly. There's no need to lengthen the hull, and what's more, some of the welded M4s didn't have the 3-piece gearbox arrangementeither. So, just carry on as you are, and you'll have an incontestable 1C Firefly!. A beaut too, judging by the progress you're making
Here's a pic of an M4 Firefly.
(I scanned this shot from my copy of David Fletcher's highly readable 'British Battle Tanks: American made WW2 tanks")
Anyway, I have Dean's Firefly kit waiting in the wings, and it was always my intention of going down the 1c road too. 
