Paul,
Don't see a replica type in your profile, so the Sabre which I am assuming is the same as the KVA is closer to 60º-62º. I think if you figure between 55º to 60º you will be good what ever the replica you have. With the restriction in the height for seats in a GT40 that range won't give you much variation in the position of the top of the seat back. Don't have my sine/cosine tables handy or I'd work it out for you.
Lynn