Dunno about part numbers from the other vendors - but Speedway has a 1/2" phenolic spacer P/N 545-64945 ($31.99). It is made for the smaller base plate carb - like the 390 CFM Holley. The bores are 1.467 diameter.Get a "Number 6" thermostat from NAPA - assuming you have one nearby. It has the larger 1-1/2 poppet opening and has a 170° opening temperature. Alternately - get a Mr Gasket 160° "high flow" thermostat (also 1-1/2 poppet size) for a big block Chrysler (also fits the Y-block) and modify it with the three extra 3/16" holes shown in this photo.
Visit the Stewart waterpumps site for an explanation of the "pressure balancing" holes. They are easily drilled at home - or you can buy the modified part directly from Stewart. My guess is that they add maybe another 5% to the flow capacity of the "Robertshaw" type thermostat (which is reported to flow better than the other poppet styles).