Honda-SOHC

I always used a light film of GE Silicone (pure Silcone caulk, not the paintable goop) on the head gasket and the O-rings for the oil passage. It was a poor design relying on the O-ring alone between the head and the cylinders. Why they didn't have a similar setup for the cylinder to crank case with the oil jet is a mystery. However I've got Yoshimura big bore kits with higher compression pistons in both my CB500's and they never leak at the cylinders or the oil passage. You have about a half hour before it starts to set too much so make sure you get the head on and torqued within that period. The silicone also makes for an easier cleanup the next time you change the gasket. Paint thinner or mineral spirits seems to slowly eat the film left behind.

1. Float height is the manual is 22mm. But this is just a guide. The tang on the floats when new are tangential to the pivot pin. However after mangling them several times, you may notice that they are no longer tangent. So feel free to adjust the float height to 23 or 24 mm. That little offset is magnified by the ratios of tang to pivot and float height to pivot. As long as your covering the jets it should be fine. Measurement of the amount of fuel in the bowl is a pain and not easily done unless you use a drain screw modified with a clear tube to view the fuel height.
2. If your having trouble with the overflows barfing out fuel, one thing for sure is the needle valve springs over time become unequally compressed and less resistant giving you all kinds of different measurements. I suggest buying new needle valve sets. You will notice the difference immediately when you set the carbs on the bench. The floats will touch the needle valves and not compress the springs at all. Also these stock Keihin carbs use a lot of o-rings which over time will not seal. Especially the o-ring on the needle valve seat. If this one is bad, your carbs will eventually overflow. These needle valves are brass on brass. Some are rubber on brass. So there is no substitute for a good spring in the needle valve.