I like the tool you found. I think you are being reasonably causious especially if it is your first time. Dont hesitate to ask questions, but try not to over think things. Speaking from first hand knowledge.
THIS. Most hardware stores have both US and metric bolts, usually you can find something. If you just "can't" then grind one down, tapered, and drive it in. You are not trying to save the nipple, you are trying not to hurt the manifoldfind a bolt 7/16 or 3/8 what ever you can get down the pipe and use a pipe wrench, heat the nipple as described, then cool use penetrating oil, then heat the casting, the bolt will keep the pipe from smashing in and will take it out.
good luck
Heat the nipple.
Rub paraffin wax on the nipple, letting the wax drip down into the threads.
Put wp housing in vise carefully.
Twist nipples out with a pipe wrench.
Watched my machine shop do it. Easy Peasy.
Sounds akin to my dads favorite line, "if it ain't broke don't fix it""You cannot break it if it's broken, but you can fix it so it cannot be fixed