Larry - re: 75mm barrels. I've heard this question come up in the past and as far as I know the U.S. only used the standard 75mm barrel throughout the war. The early version was only used on the test vehicles such as the M4A1 "Michael" that is at Bovington. Note that this barrel has a counterwright at the end of the barrel.
If someone has more info on this long / short 75mm subject please let me know and what reference there is to it.
Eric - I can hardly wait to see the end results of this "top secret" project of yours !!!
Ga-modler - Of course you can add another Sherman.
The M4A4 was not used by the U.S. It was mostly used by Commonwealth forces as part of lend-lease. If you use the rocket variant of the M4A4, it narrows the playing field down even more as it was used by the 1st Coldstream Guards.
The Marine Sherman is listed as an M4 / M4A2. Italeri is a little out to lunch on this part and they came up a little short for these specifications. But the good news is, it builds into a very nice Marine M4A3 with deep wading gear.
Happy Modelling and God Bless
Robert