Have a chat with the kit manufacturers you are looking at. They can let you know their max wheel & tyre sizes, and whether spacers should be used on their hubs to fill up that final 5mm in the guards. Forum member's build diaries also have a lot of great information.
There are several wheel manufactures that will build what you want (within a range of sensibility), but I would be very cautious of using one kit manufacturer's data about max sizes and offsets for a totally different project (if that's what you are doing), as the relationship of the tyre centreline to the wheelbearing (amongst other things) is critical for durability and safety.
Without first-hand knowledge, I would be surprised if you couldn't fit a 335/30x17 on an 11.5" - 13" rim (or 335/35x15 if still made) under the rear of most Gulf flared GT-40s, but the wheel and tyre are only one part of the engineering package.
What are you planning?
Regards
Richard