Upgrade done and I am very happy. One major departure from your post, I put the new horns in the left front wheel well because it is empty. I found a very handy frame joint with two bolts (I'd call them 1/2" but they are metric) one of which I used as a very solid mount for my home made...
Thank you so much for that very clear and comprehensive write up. I just spent half an hour with my head under the hood trying to figure out how to do this (my new horns should be here by the weekend) and your write up confirms and clarifys pretty much what I was thinking.
One thing I will do...