The difference between the housings is mainly in the bulb mounting area. The H4 bulb
http://db.hella.com.au/cgi-bin/catal...w&flmaint=2031 has a large metal round base, which measures 41.5 mm in diameter (from Narva’s drawings), and with three locating tabs extending out from the round base, two at the bottom, and one at the top, to make sure that it’s inserted the right way up. The H1 bulb
http://db.hella.com.au/cgi-bin/catal...w&flmaint=2013 is very much more compact, and measures 24 mm diameter, with a section cut out from the circle, so that it is fitted the right way up. So the H1 housing at the rear, where the globe fits in, is much narrower than the H4 housing, to accommodate the much smaller globe. You can see here
http://db.hella.com.au/cgi-bin/catal...ew&flmaint=819 , on this H1 long range driving high beam insert, that it has a fairly small hole at the rear that the bulb fits through, whereas this H4 high/low beam insert
http://db.hella.com.au/cgi-bin/catal...ew&flmaint=823 , has a much larger large black hole, although it’s partially masked by the low beam fluting. The low beam headlight buckets which are fitted in the USA would be able to fit either the standard H1 low beam, or the H4 high/low beam outer headlamps, although the wiring, and headlight plugs, need to be changed to suit the H4 bulbs.
One of the advantages of the H4 bulb is that the standard 12V globe is 55 watts on low beam, and 60W on high beam, compared to the H1 bulb, which is 55W. According to US vehicle lighting expert Daniel Stern, that means that the standard H4 60/55W bulbs put out 1650 lumens of light, where the 55W H1 bulb (only) puts out 1550 lumens. There’s not a lot of difference, but every bit helps. On the other hand, the H1 bulbs are clearly better than the H4 low beam output. If you never use high beam, sticking with the H1 low beam setup would be the way to go, but if you do use high beam, the H4’s are definitely better. The H1 low beam is always set up for bright short range lighting, with more aimed at the passenger side footpath, even when the high beam inner lights are on, whereas the H4 setup gets increased range when high beam is selected, and it’s a brighter lamp on high beam.
Daniel Stern’s description of the bulbs, as an American, is highly scathing of the US vehicle lighting regulations, which didn’t officially legalise the H1 halogen bulb until 1997, 35 years after it was introduced in Europe, and didn’t legalise the H3 bulb until 1997, 31 years after Europe, or the H4 bulb until 1992, 21 years after it was introduced in Europe. According to the Haynes 4 cylinder Alfetta workshop manual, the European (and Asian and Australian/NZ models) had H1 halogen headlamps, while the North American cars all had sealed beam, non-halogen headlamps, and from the diagrams in the Haynes manual, they had dual-filament high/low beam sealed beams in the outer lamps. While the GTV6 was a later design, it’s a fair bet that the four headlamp system was the same as the 4 cylinder GT/GTV, because according to Wikipedia, halogen lights were prohibited in America until 1978, and replaceable bulb headlights were not allowed until 1984. It’s no wonder that they were shocking lights in America, with sealed beams being decades (1940 introduction) behind halogen lights in technology! About the only advantage they had was that they were cheap (did someone also say nasty??), and widely available. Still, most of your headlamps were the same, so road safety at night obviously wasn’t something that US regulators were too interested in! Hint to the regulators – that’s why so many traffic accidents were happening at night to otherwise sober and law-abiding people – they couldn’t see where they were going!