If you use this thermostat you will avoid the problems you are having with Shewman's stats.
It has 50% greater flow space than Shewman's and closes completely whereas Shewman's only closes 80% leaving it 20% open. It is cheaper ($10.00 compared to $25.00 for Shewman's), easier to install (goes the same place as the stock one) and is more readily avalable.
It is a Stant 14157 thermostat available at NAPA as part # THM 111, at O'Reilly as a Murray 4157, O'Reilly part number 2962, or at Amazon here:
http://www.amazon.com/dp/B000C822YC.
For full discussion and test results of each stat go here:
For the Stant stat:
https://www.fordbarn.com/forum/showthread.php?t=113366
For Shewman's:
https://www.fordbarn.com/forum/showthread.php?t=112889
P.S. No need for carboard in front of the radiator with this stat.