Not sure if this is right place but...

someone told me to siphon out the old gas and replace.... Isnt there a drain plug on the

OEM tanks? Is there a reason i should siphon instead of draining? We are talking about couple gallons at most

If it's only a couple of gallons, use the drain plug at the bottom. Just make sure you have a good sealing washer to replace the old one. 

I'm about to drain/replace the tank in my 240Z. It has well over 5 gallons in it, so I have to siphon first. After I get most of it out, then it will be the drain plug.

Yes, there is a fuel tank drain on all the S30's.  I would definitely use it.

We used the drain plug, way faster and easier...it's there for a reason just like the one on your engine.  😉 

If you have an electric pump you can disconnect up by the filter and pump most of it out.

You can also disconnect the outlet hose at the tank if you want control of the first few gallons.  Easier to use Vise-grips on the hose than trying to get the plug back in if your container fills up.

I'd put a rag over the drain bucket just to see if any rust or crud comes out. Panty hose from the $ store would be good too. They'll look at you funny, just tell them your straining paint.