If you want the wood to behave like a floor should, you have to mill it like flooring is milled, with T&G edges and relieved on the backs. You can buy SYP milled as flooring in wide widths in many places. The wider the planks are, the more expansion/contraction you can expect. That is why most flooring is milled from 2" to 3.5" wide.
If you opt for the cut nailed surface nailing instead of blind nailing the tongues, you should predrill each nail hole slightly larger than the shank of the nail, to allow for seasonal movement in the wood or expect to produce checks in the wood.
This seasonalk movement can be reduced by allowing the wood to stabilze to site conditions for three weeks with free flowing air supply prior to installation. You could go one step further by sealing all surfaces with oil based poly first.

Adhesive should only be used with wood in certain circumstances. It can cause as many problems as it solves.

For a wide plank look, and your limited experience, you might want to consider engineered wood flooring of the click-snap variety