I am a definite newbie to UDS's and made the same mistake you did by first going with 1/2" ball valves. I installed four 3/4" evenly spaced black pipe nipples on the bottom of mine, but used a 3/4" to 1/2" reducing elbow and ran 1/2" black pipe into 1/2" ball valves on two of the nipples (the front). The reason was mainly cost...1/2" ball valves were literally half the price of 3/4" ball valves. I had the elbows and black pipe painted beautifully with gloss high heat engine enamel, but when I went to do my first burn, I couldn't get the thing to hold above 200 degrees without having the two 3/4" black pipe nipples with screw on caps fully opened. My plan was to season it first and then cook cupcake chicken, and I knew right away that I could never get the temps high enough with the 1/2" ball valves. I made the last second tough choice and returned the 1/2" ball valves and ate the cost of the 1/2" black pipe and went and bought 3/4" ball valves, elbows and pipe. Once installed, I had no issues whatsoever getting temps above 300 degrees, and I cooked cupcake chicken last Friday at 325 degrees for almost 3 hours and the next day I cooked two 8.5lb pork butts between 275 and 300 for almost 8.5 hours. I was thrilled with how little I had to babysit it. Go with wider pipe and avoid the headache.
You can see my build here after moving to 3/4" ball valves. You can see in the final product picture that I have yet to paint the pipe and elbows, but I put Crisco on the elbows and threads because it was raining part of the time and luckily they avoided rusting.