I doubt if it will be necessary to clean the threads in the block. When we're prepping a race engine block, I use a slightly worn threading tap with the points of the threads blunted slightly to keep from oversizing the threads in a reversible drill motor to chase the block threads. The maximum cooling system pressure you're going to encounter will be 15 PSI or so, and just about any brush-on sealer will handle that without a detail cleaning of the threads. Unless you're seeing coolant drip lines on the outside of the block, just pull and seal the bolts under the valve covers one at a time. I guess it would be possible to fabricate a thread chaser by welding an extension onto a worn-out 7/16X14 tap, but I don't think it's really necessary.
Jerry
