Nice job Andrew !!!
I started to build a similar "what if" Firefly a couple of years ago.
Using a Tamiya M4A3E2 Jumbo Sherman as a base kit, I added a Tank Workshop Jumbo Turret and up-armoured transmission cover, a Jordi Rubio 17pdr barrel, a Dragon HVSS Suspension and some AFV Club T66 "rubber band" type tracks.
I decided to convert it to a "Super Sherman" with a T26E3 Pershing turret, similar to a prototype that the Americans started to develop at the Detroit Arsenal in 1944.
As usual, it's currently lounging on a shelf in my hobby room gathering dust with a pile of other shelf queens.
Thanks for giving me some inspiration to complete it !!!
FYI - Apparently the British started to convert some M4A3's into Fireflys for the U.S. Army.
http://freespace.virgin.net/shermanic.firefly/home1.htm Interesting stuff !!!
Cheers
jjumbo