The replacement heater channels are a pretty good fit. I have decided to punch holes and do plug welds along the bottom and top edges.
Here's a shot of the channel under the rear seat. Note the top edge of the outer sill here that was not welded back to the heater channel when the sill was replaced.
Forgot to take photos during the welding stage, but here is the passenger side channel completed. Just need to dress down the welds along the door edge.
This is the outside edge of the sill. There was a 10mm gap along this edge that needed clamping together as I welded.
Should get a quick trial fit of the body on the floor in the week just to make sure everything is still lining up correctly.