Hi Andy,
Great work! The details are amazing. It's interesting to see how you've converted the /85 interior into one for a /76.
Speaking of which, it's my belief the added fuel tanks behind the firewall are appropriate for a 1942 T-34.
In the book, The Unknown T-34, I found a chart on page 163, which contains a row identifying the "Capacity of fuel tanks. The note for ** being "the capacity of additional barrels (tanks)".
Anyway, the capacities listed (internal/additional) are:
T-34, 1940: 455
T-34, 1941: 465/134
T-34-76, 1941-1942: 640/134
T-34-76, 1943: 540/300
T-34-85, 1944: 540/300
T-34-85, 1944-1947: 550/270
The bump in internal capacity in 1941-1942 is close to what the "US Army's 1944 T-34 Preliminary Report" mentions. The original T-34 manual lists 6 internal fuel tanks, but the tank being studied had 8 fuel tanks. Each additional tank holding 75 liters each, for a total capacity of 610 liters.
So I'm thinking the "additional" fuel tanks were introduced early on in the
1941-1942 series. I have a photo which shows a T-34 with the early single, engine-top, air filter. The tank also has the pair of fuel tanks behind the firewall.
Here's the picture...

...and a close up of the top of the fuel tank...

I can't wait to see more of your craftmanship.
Take care.
Karl