it is mighty low to the ground, not something you will want to drive a lot,
it is a lot of weight placed on just 4 of the support beams, perhaps bolting a 12ft long beam to every support beam and then bolting the 4 chains to that beam might spread the weight,
Is their some type of C channel under the chains to protect the tank? do you have some type of rubber pad under the C channel?
It looks like 1500lb chain, are those old grade 5 bolts? they look like 3/8in, originally they were good for about 6000lbs but if old or abused might be a lot less, are they bolted all the way into the floor or just through one thickness of the floor beam, the beam would need to probably be 1/4in thick just to match the strength of the chain, would consider putting a second nut on the bolts with loctite,
when I added my second fuel tank I tied all the weight to the main bus frame and not to the floor beams, I did not know for sure or not if the floor beams could take the 600lb weight of the tank, but I could tell that every truck and bus fuel tank I had ever seen was tied only to the main frame and not to the floor,
my bus thread, http://www.skoolie.net/forums/showthread.php?t=8860&highlight=bubb