I think that there are two connected issues here, that have different requirements and so different ideal solutions.
The issue I'm primarily interested in is to take the load off the light switches at the top of the steering column. That's because, I think, switching something like 30 Amps steady state is liable to have a bad effect on the switches, especially as they get older. I also suspect that there are issues in surge currents switching the lights on, and, possibly, back EMFs from inductive components in the load switching them off. But the steady state current is, on its own, enough to worry me.
The other is about upgrading the lights, i.e. fitting higher power bulbs. As I understand it, you can do more or less as you like in terms of bulb powers with a car from before 1982. I guess alignment becomes a bigger issue if you do upgrade, and needs some care.
The problem there is that the original feed lines to the headlamps are too small. From looking at them under the dash, all four mains are running off a 3mmsq, 30 Amp conductor, and both dips are fed with the same size cable. That's only just over 400 Watts at the maximum current capacity (when new) of the cables, i.e. 4 x 100 Watts for the mains. And I, at least, would want a bit more of a safety margin than that. I would think, if I was upgrading the mains. I'd want at least 4.5mmsq, preferably 6mmsq, for the 4 mains, 3mmsq, for any pair, and 1.5mmsq for any single lamp. Steve also wants more fuses, presumably to guard against any one filament failing catastrophically and blowing any reasonable sized fuse common to a set. Maybe that's a bigger issue with bigger or more efficient (power to lux) bulbs. So, relays are pretty much a must with such an upgrade. And, for several reasons, you want the relays as close to the headlights as possible.
But also, given that standard wiring is adequate for the standard headlight powers, if all you want is to take the load off the switches, like me (and don't worry excessively about fusing all the lights in a set together), the relays can go anywhere that's convenient. They won't necessarily change the losses in the cables, but they will remove any losses from the old switches, with old worn contacts, etc., as well as probably extending their life considerably.
My ideal solution to that would be to put the relays as close to the switches as is possible. So, ideally inside the steering column shroud. But, I don't feel that's convenient enough. So the next best place is under the dash. But I do have a reason to feed them from elsewhere than the battery side of the fusebox. So, ideally, I want a male and female 9.5mm spade to make a removable splice at the connector block on the battery lead.
OK, so 5 points here.
Point 1, save the switches, agreed, we're all on the same page with this one!
Point 2, upgrading the lights, I think you'll find anything higher than the standard H4's 60/55w output is STILL illegal, not that that stops many people. Alignment is another issue altogether, my experience says it's ALWAYS critical. Good alignment can make poor lights bearable and good lights safe for other road users. Take the bezels off before the MOT and drop the tester a couple of quid to set them up properly, it's a worthwhile investment!
Point 3, is the wiring up to it? Yes and no! I think your 400w main beam output is unduly pessimistic. Standard sealed beams are 45w each for a 180w total or 240w IF you can acquire a pair of the legendary early fit 75w inner sealed beams. The "normal" halogen bulb conversion kits give an output of (2x60+2x55w) 230w on main beam, only if you to illegal extremes and use 100/80w outers and 75w inners do you get close to that 400w figure. My insistence on multiple fuses is based on the fact (not theory) that the fuse will always blow on a dark and stormy night, miles from anywhere. If i've only lost 1 filament I can keep driving till I either get home or find a sheltered place to fix it, lose both dips or all 4 mains and i'm condemned to a soaking at the very least! I call it outwitting Murphy's law!
Point 4, keeping the lights standard, fine, if that's what you want, my only comment is that sealed beam units are growing increasingly rare and expensive. If it's a daily driver, unless you can find some 410 bulb fitment (45/40w) lamps, sooner or later you'll HAVE to upgrade to Halogen.
Point 5, powering the relays. I don't see a problem with the fusebox as a power source, almost NOTHING runs off that fuse, clock, horn and interior light. Yet it has a large size cable feeding it, straight from that 4 way on the main lead. But if you don't like it, your choice, let's consider alternatives. In and out using the 9.5mm male and female spades as you suggest is possible. They must BE available as Jeroen uses them, if you can't find them anywhere else, I expect he'd sell you a pair (and the insulated sheaths which I suspect are harder to source than the terminals themselves) My own, slightly more radical, solution is to ditch the original 4 way on the main power cable, horrible bodge that it is. This lets you use a fatter cable for more amps to the starter and then take a second lead from battery +ve to a flitch mounted insulated live post (less than £10 from CBS) to which you can connect as many wires as you like (up to about 10) via ring terminals. Finally, did you know that OHV Dolomites use a different sort of 4 way to the slant cars? On an OHV car it's a remote, flitch mounted unit. If you know where to look you can find one on a scrap car, mount it near the original and gain 3 more permant live feed possibilites with something that LOOKS stock! The part number, not that I think it will do you any good, is 150640.