those are the ones that make you wince
when you have to tangle with them....
I would go buy a couple of shark bite 3/4 Tees
and simply cut out those old nasty ones,
it looks like you got plenty of loose play in the lines
then you could run the plumbing to the water heater
with anything you wished
a ball valve is always best on the incomming cold side
and of course on the gas line you want to use a flexible gas supply connecter
It looks like that old one will probably fall over once you disconnect
it from the pipes....I would suggest draining it all the way down first...
have a fun time