Using Floor Boards for Sills, Reveals/Jambs in very thick stone wall?

I am working on a project with a very thick stone wall 550mm thick and large windows. We have timber sleepers in the rock as lintels and want a rustic look. We need to find timber to make the 300mm wide internal sill plates with 300mm wide reveals (20mm thick). The timber is on a budget and we have a tight time frame. We could get timber from the Sydney area.

One option we are looking at is to use floor boards (because they have tongue and groove) and we can fit the lengths together easily to get 300 width. It is also relatively cheap as it is volume timber. We can colour match the sleepers with stain if necessary.

I'm currently looking at floorboards like "Red Ironbark" and "BlackButt" which I can get pretty cheap from Mitre 10 (from Boral).

My question is, "What are the negatives to this?" "Will there be a huge problem with shrinkage/ movement?"

Note: Floorboards were used in another building on this property as sill plates for windows and appear to have worked well. This was Cypress Pine and stained.


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