I just did the opposite from standard to the weird merc... what I did was hang the engine over the transom and use wood shims to make sure it was level... I measured several times and then drilled tthe first hole about 1/8" with a 1/4" bit... I moved the motor and drilled that hole through with my final size bit.... I put the motor back on, shimmed it, and bolted that one. I then drilled the rest of the holes 1/8" with the 1/4" bit and pulled the motor again to finish them...
I'd recommend a helper if available..... when you are drilling it is easy to drill straight on the horizontal plane but a spotter from the side can be very helpful to guide you on the vertical plane.
I had considered a template but I had to hang the engine to decide height and in my case it was easier to just drill. Your results may vary
