Regarding the two line-three line issue, the third line is just a vent for the expansion chamber (hope I got the right name for it) that goes to the flow guide valve; don't see how it would make a difference.
Regarding the "three reasons" for "percolation", in the modification manual that was posted, reason 1 makes sense, the other two seem a little iffy. However, that information applies to the flat top carbs used in '73 and '74. I'm not familiar with how those carbs work so maybe it's correct for them.
My fuel pressure gauge (hooked to a tee between the mechanical pump and the custom fuel rail) seems to back up the percolation thing. On hot days, a few minutes after shutting off the engine, I've noticed the fuel pressure climbing slowly, past the usual 3.9 psi, to about 5 psi or higher. Then it drops suddenly. If heated gas vaporized and built up enough pressure to open the float valve, like the book says, that could explain it. I've also smelled gas after a hot start. If the float bowl is full when I shut off the engine, and the percolation effect pushes more gas in, that would explain it. My fuel rail is more isolated from the engine than the stock one, but maybe not enough.
It seems like having a return line (I don't) would help the situation, but maybe not. Maybe no fuel return line would raise fuel pressure slightly, that might help prevent vapor lock (not percolation). Also, they already had return lines on the cars discussed in the modification manual, so apparently that didn't solve the problem.
If the main cause of heat in the fuel lines is due to conduction from the bolted attachments to the engine, then it seems like insulating the lines would help keep the heat in.