In theory....you can mill off the difference in head gasket thickness from the cylinders, and then use the OEM thickness of O-rings.
In theory. But, this hasn't always worked with the 750 engine because the depth of the O-ring holes' recesses in the cylinder deck seemed to have a wide tolerance in actual production, which led to many of these engines leaking (pre-K4) oil after about 10k miles when they were too deep. We often see these engines leaking oil from those sites from the old head gaskets finally losing their seal (usually from being parked for decades) while the O-rings are simultaneously aged enough to have shrunk a bit.
I once measured some 20 cylinders to find out what pocket depth Honda might have been trying to achieve, and I believe it to be 0.6mm in the 'perfect' cylinders. Their production tolerance on such milled-in features was +/-0.04mm (or, about 0.002") while their acceptable O-ring thickness was +/-0.1mm above 4mm and +/0.04mm below that thickness. Since this particular O-ring was spec'd at 2.5mm thickness, it could be 2.36mm and the cylinder's recess could be 0.64mm along with the head gasket's max of 1.1mm (seated thickness), making the total height of the pocket (0.64 + 1.1) = 1.74mm for a (when new) O-ring thickness of possibly 2.36mm, or a squish of (2.36 - 1.74) - 0.64mm. So basically, this small squish of 27% was supposed to hold back 60 PSI of hot oil pressure with the OEM O-ring of 11 x 2.4mm.
To help back up the O-ring, Honda's head gaskets were fully impregnated with sealant that would melt after enough engine full-temperature time and [very nicely] grip the mill-marks on the head and cylinder decks to help seal it. But, the O-rings shrink with age, and faster with heat, so in time there isn't much sealing pressure left to hold back the oil. Once it gets hot and loose it also dissolves the sealant that once held it back, ESPECIALLY in those engines where the owners used oils containing detergents (don't do that to your SOHC4 engine).
To figure out how to make something better, I worked with some airplane mechanical engineers (circa 2006) where I was employed, as they are well-trained in that lifetime-testing sort of thing. The end result was that the O-ring thickness needed to be matched to the newer head gasket thickness, plus 50% more 'squish' applied at assembly, without blocking the oil passage. After searching for about 2 months I found the Parker #02-111 to be 10.77 x 2.62mm size in metric, nearly a perfect fit with the thicker head gaskets and NO cylinder milling! And, when they are fully squished with even the cylinders decked back as much as 0.020", they will not block the passages and are still not overly squished.
So, I buy them by the 100s to get them cheap and spread them out to everyone [here] to prevent heartaches.
