First, you need to bolt the engine to the engine mount. You can even use C clamps to temporary mount the engine to the engine mount. Then measure the horizontal distance between the left and right mounting holes. Divide that distance by 2, which gives you the amount you measure out from the vertical centerline marked on the firewall. Then just line up the crankshaft centerline of the engine mount (the surface on the engine mount where the engine flanges sit on) with the horizontal centerline drawn on the firewall.