577/450,
What I have done for a 12.7X44R and a .43 Mauser mod71 is cast a larger bullet in pure lead, size and lube using a sizer with the least amount of re-sizing, then resize it again with a smaller diameter sizer from the base up the shank to where it will seat far enough into the unsized shell to chamber. I have effectively manufactured a heeled bullet this way. Then running the whole assembled round through the sizer die tightens the shell enough to keep the bullet in.
I don't know just what dimensions you are working with but if you can get say a 0.459 pure lead lubed bullet then go 80% up the shank with a 0.440 sizer, you might wind up with the heeled bullet you need.