We had a heck of a time with flexplate bolts on the newberry motor we ran in the truck. We tried super loctite and everything. Finally, Dad looked up the torque specs for a 1/2" fine thread grade 8 bolt and discovered it was really high. We started torquing them to 120lbs (with locktite of course) and they quit shaking loose.
I had problems with convertor bolts on the Donovan in the Anglia. There wasn't any room in there to get a socket on them, so I was tightening them as tight as I could with a wrench. 3-4 runs later they were loose (kind of hard on flexplates...) I finally ground the ends off of some rod bolts. They've got the small 12-point heads on them with gives me just enough room to get a socket on them. I torqued them to 90lbs (good enough for rods...) and they haven't moved since.
However, I know that harmonics are bad news, and sometimes there's not much you can do to keep things together. Maybe a bigger dampener? You could put one of those big air pumps on the engine - the big belt on the front of the crank works pretty good on dampening harmonics
