My team did this with PLA for a part that supports the entire weight of our FTC robot (~12 lbs) over four screws that are roughly that size. It's been working just fine for us, but we don't have nearly as much force on it as you intend to. It was printed on a Makerbot Replicator 2x and I think we used the standard settings for temperature and layer height, but we increased the infill to maybe 20 or 30% and the number of shells to 3. I think the infill won't really matter for tapping something; the number of shells is going to determine the thickness of the plastic around the holes that you want to tap.
Here's a picture of the part with the screws in it. Our mountain-climbing mechanism goes on the top and the bottom attaches to our robot's bellypan.
https://drive.google.com/file/d/0Bzk...ew?usp=sharing