I have seen many cowlings to spinner transitions.
The one thing I can say is that it is nothing short of a bunch of work to get it to fit properly.
I have never seen one "fall into place"
The first thing you have to do is get your plane in the flying position (level) with the engine hanging on the plane. If possible you can hang your engine several months before to let it "sag".
Once in the level position, you want to shim you engine plumb. You want you crankshaft face to be perpendicular with the datum plane. A electronic leve works real well for this.
Then you can start fitting you cowling with the prop installed and the spinner installed.
This will get you to a rough stage.
Then the real fun begins with the final gap fitting and height fitting.
I hear lots of people building their cowlings to allow for sag. You don't want to allow for sag. This means you engine is off it's natural thrust line. This equals loss of MPH.
I would check to see where you crankshaft flange is at compared to you datum. Then make adjustments for that including fiberglassing if needed.
I know I'm going to get some flak for this post, but the results is what you are after.
My RV-6 pictured has 180hp-that's it. No high compression pistons, electronic ignition....ect. Bone stock. I have beaten RV-8's with 200hp before and it doesn't make them happy. Top speed with 4-way 2 GPS's average is 224MPH. Confirmed many times over. And this is with the standard prop as you can see in the pics. Not too bad.
The way you have your engine sitting on the nose helps in getting the MPH up.
Fitting the engine to your plane, then the cowling to the engine. I have to adjust my engine back up after about 75hrs of flight to get it back square to the datum.
I agree with the other post that if you just want to go fly, then see what it does and worry about it later. You can alway readjust with fiberglass and lot's of work.