It sounds like you have everything right. On the plastic spacer they may found it warps or melts because on the street the temps under hood are higher than at the track. You get the motor up to temp in the lanes, make a pass then pit and open the hood to cool down for the next pass in an hour or more.
 
I still say for S&G lower the level, I know thay say it should be half way on the glass, but I do not see what has changed with the carbs that they can up the level and not have a issues with something else.
If yo ulower it and it fixes the shut down / boil but causes something else sneak up on the level till you can live with it.
Dave ----

I called Holley tech support today. The fella there said either I have a leak or the fuel is boiling. Since I only seem to have the problem after shutdown, fuel boiling would be the likely suspect. He said what I've done so far is pretty much what could be done to correct the issue. Since it hasn't helped, my choices are few. He said the secondary bowl is normally the first place for this to show up as fuel sits there not being used more than the primary, therefore has more time to preheat. He also said the ethanol blend fuel has a lower boiling point with winter blends showing signs of boiling at temps as low as 100 deg.
  I could modify the fuel system to recirculate off the fuel connector at the carb, using either a fuel log with a "pill" return or making up my own using a fitting like this: http://www.holley.com/17953NOS.asp . That would mean I would need to remove the fuel return from my RobbMc fuel pump. Or run an electric pump and use a return regulator. That would mean modifying the fuel tank for a sump pick up. Either way requires new plumbing for the return line. Not something I would look forward to as all my plumbing is currently stainless steel, and it's hard to run new lines neatly with the car assembled and no lift.  
  Otherwise, I'm not sure if switching to an Edelbrock carb would make a difference, or spend several thousand dollars to install EFI.
 
Dave Z

Quote from FuzzFace2 on 05/19/12 at 23:25:27:

Dave,
  Man, I told you there was a difference but you just didn't want to believe me.
 
  Return is off the RobbMc pump as stated earlier.
 
  Electric pump allows for a bypass regulator to be used and it could be placed closed to the carb, regulator cannot be used with a mechanical pump due to the pulsing. The three port filter is 5/16", the rest of my fuel supply system is 3/8". I didn't want to add a restriction and the adapter fittings.
 
  Fuel return from as close to the carb as possible would keep the fuel less prone to sitting and picking up heat and therefore dilivering the coolest possible fuel to the carb.
 
  Mechanical pump is bolted to a heat source, the block. Electric pump is not mounted to a heat souce.
 
  I currently have two gaskets between the manifold and spacer, spacer to spacer, and spacer to carb. Holley tech said the phenolic spacer I have should have been sufficient and adding more insulator wouldn't make a difference in his opinion.
 
  I thought about the studs and nuts as well, but putting my finger on them when I have the problem they really aren't that hot.
 
  At this point, my son wants to ditch the carb and go with EFI. That's his choice, I'll be keeping the carb for another engine though, I still need to find a solution at some point.
 
  Dave, thanks for putting in the effort and responding. I just find it hard to believe I'm the only one with this problem. Either anyone running a Holley on the street is using race gas, low boiling temp gas is only sold to me and my son, or there are not street driven cars with Holley carbs.
 
Dave Z

Finally a breakthrough. (or not) After pages of Google searches where people had the same problem that always gets blamed on the fuel bowl overfilling and never gets resolved, I found one case where someone suggested to check the air bleeds. If they are not open, fuel can syphon into the ventury. That seemed consistant with my problem. Keeping in mind the fuel may still aggravate things.  
See the picture below, the air bleed is not drilled completely. Only one side though.

The carb is probably out of warranty by now, should be interesting to see what Holley has to say.
 
Dave Z

Problem solved. I contacted Holley, they said send the defective carb back. When they recieved it, they sent a new one out. Carb is on the car and problem looks to be corrected. After it gets driven some, I'll start undoing my other fixes. The altered fuel line and the phenolic spacer.
 
Dave Z