Fixing an indoor bench with green oak

[NOTTNICK]

I am about to strengthen a lovely old indoor bench that sags in the middle when sat on. I am intending to use a piece of 75 x 50 oak (2 metre length). I can get some green oak (probably air dried for a year or two) that I can plane up for a lot less than buying a dried planed board.
Do you think this would be risky? It'll be screwed / glued tightly to the underside. Or should I accept that I'll have to pay a lot more?

 

[Stigmorgan]

I imagine you will get a lot of movement and shrinkage as it dries

 

[NOTTNICK]

I imagine you will get a lot of movement and shrinkage as it dries

That's what I am thinking.