It had to be there to make the casting I think.

Aluminum is a bad choice as it will conduct more cam oil heat into an already heat soaked desighn.

I like sealing them with right stuff on either side of the gasket, it is a little flexible and very strong and since Al and Steel expand and contract at different rates it needs to be able to move, the gasket alone won't last long in that environment.

Also just welding on a flat plate will reduce hp by moving the floor of the plenum closer to the mouth of the ports, the steel pan has a step to help aleviate some of that.

If its just an old piece of junk you dont care about and you have some sheet Al laying around you don't mind wasting then by all means weld away. You will want to make sure it is thick enough it wont flex and fatiuge and crack, 18" vacuume over that big of a surface is a pretty powerfull force to be going on and off constantly.