If you want to get a positive supply from a negative, then you want to use a buck-boost type.
It will generate a node at opposite polarity (relative to zero ground) from your supply. The buck or boost type will not do that.
Screenshot:
I can't say how this arrangement will cooperate with the LTM 8027.
You may need to invert the control signal.
Or experiment with places to put the sense resistor. Etc.