1968 GTS 340 with Carter AVS 4966S

The thick gasket is a base plate heat riser gasket ( I know there is a better term for it but cant think right now)
, It is designed to help keep heat from the carb and improve drivability.

If you have been driving fine without it is would be safe to just use a correct base plate gasket. If you have had vapor lock or fuel boiling out of the carb between stops when the engine is how then using it might help with that.

Something to watch for is the choke pull down length. as the riser adds height to the carb the choke pull down might need to be in a different hole of bent straighter