40 Gallon squarebody gas tank... wil it fit?

[Nitis]

AS I get closer to assembling my list of things to acquire to do my LS swap I come to a cross roads with the fuel tank. I want to go inside the frame rear style for sure. Budget wants me to use a square body style which means some fab work to the cross rails under the bed no big deal.

I have a long bed so will a 40 gallon fit? the 25 just seems dainty and the 31 would be ok but why not a 40?

Any other suggestions?