I can recommend the books referred-to above. I have both volumes and the associated scale plans pack. I believe I got them on eBay.
They pre-date the Italeri kit and use the Trakz (A)4 conversion for the (A)1. But the detail photos of the real thing, inside and out, are useful nonetheless. I have the Verlinden interior set too, but not having built it yet I can't comment on the accuracy.
All of these are very hard to find now, unfortunately. The books were written by David E Harper and published by Letterman Publications in 2003. There are no ISBNs on them. The Amtank volume's catalogue number is LP008.
I've been meaning to get the Squadron Signal book on LVT's but haven't yet. I do have the Osprey New Vanguard Amtracs book, and that's no help. It has only a very basic semi-cutaway of the (A)4 interior.