1967 273 4bbl with Edelbrock 500 CFM Carb

Couple of comments/suggestions.
- the Dual Port intake has small pri runners [ they are connected to the pri t/blades ]. They have high[er] velocity than conventional intakes & will make more tq at low speeds. So when the engine goes from N to D, it might be making [ as an example ] 70 ft lbs, where a conventional intake might be making 55-60 ft/lbs. You are going to feel that as a hard shift.
- unless the engine has a 'cam' in it, you should be able to lower the idle speed to 600, maybe even lower & reduce the shift harshness. Lowest rpm without stalling when braking.
- adjust idle rpm with the speed screw & mixture screws. Have the mixture screws about equal in/out. Adjust the mixture screws for highest rpm in gear [ wheels chocked ]. Do not use any other method, it will not work. If rpm is incorrect, change with speed screw & final tweak with mixture screws.
- pri jets/rods may need to be leaner with that intake.