Should I make the swap to G3?

Honestly, the main thing that talked me into the G3 swap was just having a newer engine with all the tech and design updates that have come along with it. You can typically make good power on a modern engine with little work. Your 408 sounds like a blast and something that has been built up pretty nicely, but in the end it's still "old tech" to some degree. Kind of like you are seeing with the oil leak, it feels like the aftermarket doesn't really improve old designs as much as just keep building them. Modern engine are all o-rings and better tolerances and generally more efficient right out of the box. Efficiency isn't just about using less fuel either, it means getting the most out of what you put in, which means more power for the same fuel input.

In my mind, getting 400 hp out of an old style smallblock is not near as hard nowadays, but starting with a modern engine you're basically just a cam swap and a tune away, plus it won't leak and will also tend to be more drivable throughout the whole rpm range and now just a top end number thing.