we are thinking about replacing some single 18.4x38 tires on a 1600 gallon tank that we pull between our tractor and field cultivator with some old floater 66x43x25NHS 6 ply tubeless tires.  I have found a set that a neighbor has.  They have been stored inside, they are on rims that I think would fit our 10 bolt pattern for mounting.  The haven't been used for about 15 years.  They have about 50-60% tread left with very little weather checking, stubble damage.  The sidewalls look fine.  There is also a 3rd tire with no rim with a lot of road wear, maybe 10% tread left.

What would be a fair value for these tires?  Is the age of the tires a deal killer? They were used when my neighbor quit using them approximately 15 years ago.  They have been stored inside, up in a barn loft.

It looks like we would have to extend our axle about 4" on each side for clearance between tire and tank.  Not a big deal, but some work.  Would I be better going with a set of 18.4x38 band duals instead of the floater tires?  Would the band duals be heavy enough?  We apply 32%, so 1600x11#/gallon is 17600#.  Would these floater tires be heavy enough?  Is 6 ply on the lighter side?

We are trying to eliminate some compaction.  Going back to using NH3 or urea spread or 32% spread by the fert dealer isn't really a option in our situation.  Thanks for your comments.