Drill out the mudguard brace if you're binning the old guard Ted. You can actually use the brace and fit a different blade off another Honda that uses a 19" front wheel, I used the blade off a CB900F from the early 80s, 900FZ being the best one, the FA and onwards had a air scoop incorporated in the blade to deflect air onto the oil cooler and this can cause problems when fitted to a 500/550. The upside is that corrosion is a thing of the past being ABS plastic, the downside is you need to get it sprayed to suit the colour of the bike.
Even if you don't use the brace it can be useful to others who are planning to do something similar.
Pic of the guard, as the bike was being restored, guard still in silver

And after being sprayed to match the bike, the guard is still pretty much pristine even after 40 odd years but the front was an experiment that Dream Machine who sprayed it really mucked up. I intend to have it repainted to get rid of the front part.

Just remembered that the pic above was taken in the street Mick Hucknall (Simply Red) grew up in.