Matty , when I got the St T in 1987 one of the things I did as part of it's initial freshening was to change over to Hynautic steering . I've never run it with cable steering .
Back then the stuff was a lot cheaper than it is now even though it was in a different ballpark price wise than a new cable and helm unit.
No interference with the engine whatsoever and you can drive hands off the wheel at any speed with no feedback . It does, however , take a little more input hard over to hard over than a cable setup.
Possibly worth considering when you figure out how long it could take and how much it could cost to get a p/s system that you're happy with . Then your water pump will have all the unfettered real estate it wants .
Just because something's old doesn't mean you throw it away !