Simon,
I have just been looking into this, as I want to build an 1:35 scale model of the
Indian Pattern FAT. No so much for the modelling itself, but as so little information is available, I'm trying to sort of re-engineer it.
Italeri kit no. 240 is a Chevrolet Gun Tractor, and I've been told it's quite a good kit. Check your local modelling store to see if they have it. The same kit has been brought on the market over the years by several manufacturers, including Peerless/Max, Italeri, Airfix, Bilek and Zvezda.
Hope this helps,
Hanno