Eindecker:

That plane is so close coupled that three degrees might turn out to be just the starting point.

I'd install the firewall with the three degrees right and down, then offset the engine mount to center the prop in the cowling.

A nice thing about planes like that is being able to have the prop not quite centered, and still not have it show too much.

So if you have it centered with the 3-3 angle, any adjustments wont be terribly apparent.

Have a good time with it.

Bill.