I'm with John on this. I've done many T&G pine floors. Rent/buy a proper flooring nailer and use galvanized T cleats, they have excellent holding power. You can find nailers on sale for around $100 and just sell it when you're done - beats renting and being rushed. For locations that you can't get at with the nailer, pick up a box of galvanized spiral finishing nails and pound them in by hand. If you insist on going the brad nailer route, I wouldn't use anything less than a 16 gauge nail and use PL. As someone else pointed out check the condition of your joists.