As for not triming a fan, my guess is that if done badly, like not cutting the exact same amount off each blade, you will end up with an unbalanced fan. That would be bad for several obvious reasons. I also am lead to believe the contour of the tips plays a major role in how well the fan pulls the air. Unless that is replicated on the shortened blade, it may not function as it should. Since the authors cannot control the quality of what we others would do, they err on the side of caution.
As for the horn hole blockoff plates, I do not have pics or drawings to share. I can tell you I made mine out of rather light gauge sheet metal, similar to what is used in duct work in homes. I mounted mine on the front side of the opening, so only a flat piece is needed. I only attached it on one side, using the two unused tackwelded nuts in the radiator frame opening. I figured at speed the air pressure against it would hold the other sides tight enough for all practical purposes. The metal is stiff enough to sit pretty tight against the opening as it is. You could also use some silicon adhesive around the opening if that was deemed desirable.
I was surprised how much longer I had to make the horn mounting plates, do not skimp on that. But too long and the harness wires will not reach! You need to drill holes for the two electrical wires to get to the horn. Big enough to get the connectors through, then put grommets in for vibration protection. Just splitting the grommet on one side will let you get it in after the wires go through.
You will need to drill two relief holes to allow the radiator mounting bolt tips to pass through the block plate. I say just get some poster board/paper and make up a pattern. It will also help you determine where the ledge for the mounting bracket slot needs to be cut. Just make a pattern, bolt it to the two unused nuts (using very short bolts!), press hard to mark the two relief hole locations, and draw a line on the back side where the horn mounting ledge is. Worked for me anyway!
Cheers, Gene