The tank can be anywhere after the PRV on the cold side. For replacement, it is a little neater to hang it and use a valve on the T so you don't need to shut the whole house off. You need to support the pipe either side of the T or use straps on the tank. You want access to the air valve. If you don't have a water pressure gauge, once the system is up, you can use that to check the system water pressure using a tire pressure gauge. Precharge the air tank before installation to the working pressure (they usually come with about 40#). Because it isn't a very big tank, be careful when pumping it up to avoid overcharging it.