Say you had access to a tig welder and could weld the bottom of the plenum on. would that be a better alternative than bolting on a new plate and gasket? Has anyone done so before, had success?

 

At cost of ruining the manifold, no.

 

I think it'd be a good alternative if you could do it.

 

I would think that it would depend upon your definition of "better".

A decent gasket, installed properly and using bolts of the correct length to mount the pan to the manifold, should last as long or longer than the engine can go between rebuilds (on a normally aspirated engine). It's cheap and relatively quick once you've got the manifold off of the engine.

A weld job, if done properly, will last longer than the engine but will cost more. It will also make it more difficult to clean up the manifold should it become necessary, as when, say, a stuck PCV valve spews oil into the intake. If you've got a solvent tank that you don't mind fouling, it might not matter -- but if you would rather wipe up what you can before going to the solvent washer and then the tank, you don't want a permanently sealed base on the manifold.

 

I thought it wasn't possible to weld the steel belly pan to the aluminum manifold because of the different metals.

 

I'd assumed it'd be an aluminum belly pan he'd want to weld into place.