The bearing surface on the 69's BTHP is going to be much less per grain of weight than with the 55. You will have to push them pretty hard to get the velocity up enough to offset the lost stabilization with the heavier, less bearing bullet. I am not sure there is enough room to do it with the case and a 1/12 twist. With normal loads, the 69 is just not going to be able to spin fast enough to stabilize completely.
I hope that makes sense, at least thats the way I understand it.
Later,
Marcial