I'm going to be doing it the same way. all at once is always best, but reality is, well, one piece at a time. If you buy more than a gallon, get an extra can and blend all the paint. if you buy 2 gallons, get an extra can. pour 1/2 into the empty can, then pour 1/2 from your other gallon into the can, mix, then pour back. follow this process and you will have uniform paint. As long as you don't paint 1/2 the truck, let it sit in the sun, then paint the other 1/2, you should be ok. It may feel like wasting money, but always buy more than you will need. cause you will need it. especially if you are like me and don't paint but one truck every decade. try and have similar environmental days when you do it. humidity and temp. I wouldn't hesitate on a solid color, metallics and pearls or candys, one day, one shot. i've heard horror stories. but it sounds like you are shooting a solid color. Make sure your prep is done and everything has been test fitted. nothing worse than having to massage a door or a fender to fit once everything is nice and shiny