Ok, so the way I see hemalurgic transfer is like this:
1. Spiking the donor takes a "scan" of their *entire spiritweb*. The more/better/fresher the blood, the higher the resolution of the "scan". Thus, the heart is a very good place to spike someone.
2. The placement of the spike in the recipient is absolutely crucial, in that it determines which point on the spiritweb you're grafting. The spike holds all possible points, since it's a complete scan, but the point on the recipient is what determines which specific Connection you're trying to make.
3. The metal is also crucial, in that it is what channels the investiture between the two spiritweb points. You can't put a steel spike in a bendalloy bind point and expect it to channel the Sliding ability (although you might get a weird result if you used tin or pewter). Atium creates a superconducting channel to the point (especially if it is pure), and Lerasium, being what allows allomantic powers in the first place, would simply grant all of them no matter where you put it - although there's probably a "best" place on the recipient.