Gray Water Tank Instulation and Support Beams???

[coairstream]

I have a 1972 Trade Wind that I'm looking to install a Gray water tank. I removed the belly pan and have the underneath exposed. The current support beams look like they are pretty rusted so I think I will cut them out and replace them while installing the tank. My question is the tank is going to be rather large and the cross supports are going to be in the way. What do you think about relocating the supports as to accommodate the tank or should I have the tank fabricated to accommodate the beams?

Thank You