I always ran the wire in the heads and left the groove in the block as the receiver groove.

The reason is you can get the wire very close to the edge of the chamber (especially with some wedge heads) and the wire helps support the groove. If there is no wire in the groove and it gets a little hot in that area it can fold up quicker without the wire in it.

I always grooved the block, put some rings in the block that weren't butted up and then put some bluing on the heads. I'd then bolt the head down and it would leave an impression where the groove should be.

Then up in the manual mill, indicate everything in and center the tool holder that was made to be used in the mill and cut the grooves.