I noticed that the factory did not use gaskets on the exhaust manifolds of my L-69. The manifolds were just bolted directly to the heads with nothing in-between. Do you guys here recommend using gaskets when re-installing them on a replacement engine?...

gaskets should not be needed provided the surfaces are clean and straight. rust and pitting on the manifold and/or a suspected slightly warped manifold may benefit from gaskets.

I always install them; just my preference. Especially if the engine is out of the chassis. It's easier to put them in while on the hoist or engine stand rather than finding out you need them after the engine is installed.

I go without. Gaskets introduce 6 new opportunities for a leak.

How would they leak?...

I use the gaskets on manifolds and have never had a leak. A trick I have learned, it works on both gaskets for factory manifolds and gaskets for headers. I heavily paint both sides with VHT exhaust paint, color dosent matter, then install them before it dries, the VHT acts almost like a sealer. I know it sounds odd but I have done this countless times on the medal type gaskets and they never have leaked!!