[vettestr]

I swear by Permatex, Aviation #3 sealant for freeze plugs and bolts or studs that go into water/coolant. Use Aluminum plugs so the expansion rate is the same would be helpful. You mentioned you have chased the threads already in the block but how about checking the threads and pitch on the plugs. Determine the bore size and thread interference using a plug with blu-Inga or dye.
There is a ceramic coating in liquid form that works well to seal a block around liners, plugs, etc.. Another idea is to clean the block and plug with a chemical used to clean aluminum before welding, it is a liquid that will foam a bit as it etches and cleans the surface from earlier attempts to seal. Good Luck - you will get her to seal up. Keep the faith