I don’t have a 455 GL but a 365 GL you are right to be concerned about the space available. I have replaced some of my sea-cocks with Tru-Design and they are much better but you need to measure the carefully. You may also need to fit them in a “different” way. For example I found there was insufficient room to tighten the internal nut; instead I made a tool to hold that still while I turned the through-hull from outside. That also meant I had to ensure the angled part was in the right position.
Key is to test it all without any adhesive/sealant first so you can see how to achieve it.
I plan to change some more this winter but recognise I will not be able to use Trudesign on all the sea-cocks.
There are various discussions on this already in the forum plus https://www.dufour.org.uk/dufour-news-111/