Permatex makes a gear oil rated liquid gasket material that I've had good luck with for the rear cover. It applies like silicone, but makes a tougher gasket that doesn't get degraded by the oil. The front cover paper gasket can be held on per Griggs suggestion, I've also used the gasket maker that Kubota sells for many applications on its tractors. I'm sure there are other equivalents out there. It is very thin and can be brushed on. It sets up as a stiff rubbery gasket that makes an excellent seal but is paper thin. It runs and makes a mess if put on too thick.
As far as greasing the gears, if your really concerned, before you drive off jack up the rear and turn it over by hand a few revolutions to coat the gears with some oil without loading them. But unless your doing your rebuild on the starting line of a drag strip, easing out of your workspace to some normal driving will provide plenty of lubrication within a few feet of travel for normal break in.
