All good modifications, but none of them will help reduce noise in the cabin on motorways.
The main thing I've found in terms of noise on my car is down to wind noise. First thing I noticed was that there was a gap between the inner edge of the door seal and the bodyshell, so the wind noise around the top of the door was quite bad. For a temporary solution, just getting some self-adhesive foam tape (draught excluder, basically, but closed-cell foam is best as it doesn't take on water) and sticking it around the door aperture is enough to fill the gap and reduce the noise. Proper solution is to find and fit new door seals.
The other thing I've thought about is making some clip-in "deflectors" to sit in the windscreen pillar and smooth out airflow around there. If you look at Silver Bullet (and I think the HC saloon that Vauxhall Motorist modified for aerodynamic testing), that has filled-in pillars so the rain channel isn't as distinct as it is on normal HCs, for that reason. I don't want to make a permanent change to my car, but I thought that maybe a bit of firm plastic that could clip between the edge of the drip rail and under the side of the windscreen rubber might do the trick, and can be removed when not on the move.
Failing that, a better stereo perhaps?
I'm not making light of it, though. Some people are more prepared to put up with a noisier car, either because they're used to it, or because they embrace the feel of a classic, or whatever reason. I agree with the OP though, if I'm going to go any kind of distance I'd like to try to improve the experience as much as I can. I spent a bit of time trying to make my car nicer to drive in the hope that I might drive it more, I've got a bit more sound-deadening on the bulkhead for example. But you have to remember that these are old cars, designed in the late sixties (or even the mid-sixties as most of the suspension is effectively HB) and they're never going to be the comfortable mile-eaters that a modern car is.
Dan, on the aux water pump, is that fitted in one of the hoses? My modern has something like that, to keep coolant pumping around the turbo up to fifteen minutes after the engine is switched off.