The only time I have had to remove them they were litterly falling out.
I drilled a 1" hole with a hole saw for 1 new location but I think several 3/4" holes in addition to the existing ones should be good.
If you want to remove the existing plug welds I would use a torch and be real careful as you melt out the weld.
To drill new plug weld holes.
Just take a regular hole saw with the pilot bit in it just set real short. Drill untill the entire holesaw is cutting into the cast but the pilot is not into the tube yet. Then remove the pilot bit and run the hole saw slowly in the groove it started, this should keep it lined up and not let it walk around on you. The chunk of cast should fall out and let you get a nice new plugweld in.
