Thermostat with cool tin and 1 5/8 merged exhaust

I've got a stock thermostat setup on an engine I'm putting together for
a type 1 with cool tin (no lower sheet metal) and 1 5/8 merged headers
(no heater boxes). Will this compromise the proper functioning of the
stock thermostat? Thinking about it, it seems that the original VW
engineers used the lower sheet metal and the heater boxes on the sides
to create a plenum that would properly direct hot air coming down from
the cylinders to pass over the thermostat bellows, causing them to
expand at exactly the proper rate to maximize engine life. Do I have
to pull the cool tin and use the stock lower sheet metal and fabricate
"sides" for the lower sheet metal to properly direct the hot air flow
past the thermostat?
Thanks,
Larry H

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