I am not an expert at all, so take what I say with a grain of salt.
I'd reroute the gas line and the sewer drain line.
Then sister as many 2 x 10 (or whatever the width is on your house) as possible.
I did a 300 gallon, with 75 gallon sump on my first floor. It is sitting perpendicular to the joists. I was lucky to have a steel Ibeam which reduced the span of the joists to about 5 or 6 feet. I crammed as many sister 2 x 10s in as I could. So every joist now was 3x or 4x. Then use blocking to tie the joists together (this also helps spread out the load. It's been a long time, no damage to the floor, and the 2 X 10s are pretty cheap (I think I spent a little over $100)
Oh, you need to add the sister beams BEFORE the tank. If you do it after the tank is filled, the orginal beams are already deflected and carrying the load.
Keep this in mind for any structural beefing up you do.. It needs to be done BEFORE the tank is filled.. Ideally, you want no load on the floor at all.
Looks like the gas line is the line coming in from your meter. It shouldn't be too expensive to move the gas meter maybe 4-6 feet (away from your tank).
Call the gas company and at least get an estimate.
Again, this is all my opinion, I am not an expert..
