Mount the engine on its mount, then measure the distance from the back of the engine mount to the back of the propeller. That distance is "y".

The thrust angle is "theta".

Then, the offset that will center the propeller is:

x = y * TAN(theta)

The distance along the fuselage center line from the back of the propeller to the center of the firewall will be, call it "z" :

z = y / COS(theta)

I would measure y, calculate the other, then make up a small triangle template and hold it up to fire wall just to double check that everything looks right.

I have a little diagram here that may or may not make it clearer... but I haven't had any luck getting it scanned and attached to this message (it's only 39K, but exceeds RCU's 700x700 limit). If you want it, I can attach it to an email. PM your email address and I'll send it. No problem.