Tally:

The off-center markings for the engine mount are to make the propellor end up centered in the cowl with the thrust line pointing to the right. If you check with a square you'll find the firewall is not at a right angle to the fuselage center line, it's probably enough to see when you look down from the top. If the engine mount were centered where it attaches to the firewall the prop would be stuck off to the right.

And you don't "Build" an ARF, it's built when you get it. You just assemble it.

And then enjoy it sooner, with the hard stuff already done for you.

Bill.