You will need a fabric that isn't directional if you use the 3 1/2 yards as as Dunster mentions above.
The seam will run horizontal (side-to-side) instead of vertical. This works fine on small prints. It can be harder to match designs on larger prints so that's a consideration.
Be sure the selvages on each piece are oriented in the same direction. I use a sticky note to label the pieces. This keeps the gains running in the same direction.
If you choose to use 4 1/4 yard, add a piece to one side of the yardage. You will have ONE full piece of fabric 76.5 inches long.
The second piece of fabric is also 76.5 inches long.
Cut the SECOND piece down the length by whatever extra width you need. Then sew together using a 1/2 inch seam allowance down the length. You will have a seam going vertical (top to bottom).
BEFORE CUTTING Backing: Check with longarmer for how much extra fabric they require for quilting.
Hope that helps!