First of all, you are dealing with a natural gas connection. Be careful.
Second of all, you are discussing filling in a "hole" in the floor, and I know that the picture I have in my mind of your situation is not reality. there is no way for someone on this board to give you specific instructions on how to do what you want, because it is dependent on what actually exists here. My best recommendation is to have a professional contractor look at the job and give you recommendations that you can either follow yourself, or hire him to do. Make sure you pay him for the consultation.
To find a pre-qualified contractor in your area at no cost to you, contact www.improvenet.com and you will get up to 4 contractor referrals by the end of the day tomorrow.