1968 GTS 340 with Carter AVS 4966S

Good carb! I would NOT use the open spacer, use the thin base gasket.
Original jetting for the 4966 is 089 pri jet, 095 sec jet. 0655/063/058 rods

Original 4425 jetting is 089 pri & sec jet. 067/065/055 rods.

I wouldn't worry about the different jetting. These carbs were jetted for the fuels of yesteryear, try it & see. Edelbrock jets will fit in the secondaries [ not primaries ] if you want to experiment.