Look at this information.
1,
http://www.mar-k.com/catalog/pdf/instructions/bed_wood_dimensions.pdf 2,
http://www.mar-k.com/catalog/pdf/instructions/spacing_bed_wood_boards.pdf When I did my bed I had to determine board widths, nothing stock was right as my bed was made wider to accommodate the duals. I did call the folks at MAR-K and even though I didn't get boards from them they were very helpful. I did get the metal strips and fasteners from them. Your bed should have a "step" on the front and rear of the bed frame. The depth (top to bottom) of this step will determine the thickness of your boards.
Yes southern yellow pine may be difficult to find depending on where you live. Take a look at my pics in the gallery and if you have more questions (I) we all will help. Heres mine.
https://www.stovebolt.com/ubbthread...rn_yellow_pine_Bed_wood.html#Post1111848
