Not enough information. Is there any right angle built into that firewall or is there any down angle built into that firewall and is there any cowling with a nose ring around the propshaft?

The easiest method is to measure how far from the firewall to the back of the propellor needs to be and go ahead and mount the engine to the mount with that distance accounted for.

Now just slide the engine/mount around where it needs to be so the prop shaft exits the nose ring where it needs to be and mark the firewall holes here using the whole engine/mount unit as a drill guide.

The Great Planes hole centering tool is perfect for this marking job and I highly reccommend it.

John