I removed mine and didn't use a welder.
There is a product called Devcon which is a two part Aluminium paste.

I used the Devcon to fill all the holes not just the end ones. Then was able to sand the surface back so it matched the ports.

Just another way to skin the cat i guess.

My car is due back from the tuner this week (New engine and Microtech LT10s installed). probably a good month before i get the power tune done so no figures as yet.