So there are no centerline marks, no pre-drilled mounting holes, nothing?

You could easily find the center and make some lines there. Since it has right thrust though, you will want to move the mount to the left a little. How much you do it depends on the thrust angle and distance of the prop hub from the firewall.

You can also use your cowl as a rough guide to help get the engine positioned vertically. With a little trial and error, you can probably get it figured out pretty close to where it needs to be.