I ended up using a 5/16 bit even though it called for an "R". Left the engine running throughout the drilling and tapping with the exhaust about 3/4 taped off at the tailpipe for added backpressure. Couldn't have worked out better. Only advice I have that I didn't see mentioned is only run the tap down about 2/3 of the way, also when tapping be sure to back out and clean the threads when you start to feel more resistance. Pretty confident nothing got inside the manifold as I could feel the shavings and chips hitting my hands the entire time. Didn't even need gloves it wasn't all that hot.