As Bryan says the more oil the better both for cooling and time between oil changes. If the new tank is a standard CB750 one then you should be OK but if its smaller than standard I would reduce the time between oil changes.
Another thing to consider is the scavenge side of the oil pump has a greater capacity than the supply side to ensure the dry sump is dry, this means the oil returning to the tank has air in it. this froths out as the oil sinks down in the tank. Too small an oil tank may give a risk of the oil still having air in it when it hits the supply side which wouldn't be good.
As for the vent, its there for a reason! I have never tried this but I would imagine the tank might pressurise a bit and this might be a problem with the air in the oil not being able to bubble out so easily perhaps. Think it would be safer to fit a vent, perhaps in the cap? Try running the bike then immediately removing the cap to see if there is any pressure there, if not, perhaps the cap already has a vent?
