I tinned the flat on the rib I had filed, didn't bother with the pipe. I then made sure the pipe was properly prepared for the solder (very clean, plenty of flux applied), clamped the rib and pipe together and soldered the two together.
The brass shims you see at the end of the pipes are for centering the "ramrod," in this case a steel rod, in the pipes. At each end you see clamped to the rib heat sinks, this prevents the heat from effecting the next pipe over. I also used wet shop towels to aid in dissipating heat, protecting adjacent pipes. Also; that jar with the cloudy liquid is a homemade pickling solution of vinegar, with a tablespoonful of baking powder, which I preheat by simply dropping a short length of red hot bar steel into prior to soldering operations.
Hope that helps.
Add; I then riveted the rib to the barrel.
dave