KO - that is huuuuge! I can only relate to my own experiences - although these are on a much, much smaller scale.
Firstly, my in-laws had a large hedge of mixed planting, in which were several ceanothus. They (the in-laws) worked on the principle that "low maintainence" is the same as "no maintainence" so, inevitably, the hedge as a whole and the ceanothus in particular, got out of hand. I got stuck in with loppers and secateurs, cut back after flowering, gave them a good watering and a good feed, and they came back as good as (well, better than) new.
I did something similar when I moved into my current property. In fact, it was in an awkward position, so I was quite prepared to lose it. But a little bit of TLC and it's fine.
Neither of these was anything like the size that you have. Unless there is a better opinion from someone more qualified, I would suggest that you make haste slowly. Think about reducing the size by no more than one third for the next couple of years. Don't try it atm because the heat will put it under stress. But carefully cut back in the normal pruning mode (ie above a smaller branch or node) and cutting out any crossing branches. Water copiously and give it a good feed.
I would think this might work, but I am not qualified, so there may be better advice out there.