On mine I ended up making a custom lower mounting bracket, but for years ran it with a combination of dynamo bracket plus spacer tube. I think you can buy the correct bracket for an alternator (Sue Miller ?) as it was a Ford part. Sue certainly sells the correct length fan belts as well, I bought one at the end of last year from her.
Not wishing to teach granny, etc, but on my 17ACR the rear lower fitting had a small tube in it. Apparently this was designed to move slightly as the bolt was tightened so the correct procedure was to tighten up the front upper & lower mounts and then adjust/tighten the rear lower mount. Doing it in reverse order could, apparently, snap off the front flange.
I don't know if things have changed with modern 17ACRs or even if the whole thing was one of those mechanics myths, but I'll mention it just in case - wouldn't like to see you with a brand sparkly new but broken, alternator !
Brian
(ps - kudos on changing the wiring ! You can also buy the correct plug to go on to make it even neater - after 30-odd years I bought one from Ebay
)