750 Edelbrock issue

If you have black sooty exhaust that makes your eyes water then the carb is too rich at that stage (and/or you may need to advance the timing). Changing might help but that's really guessing. Your going to likely tune whatever carb you put on.

Any more details on your 340's build like cam, comprssion, intake, porting, etc? Current timing initial and total and when, Gears, Converter, etc? The more we know the more we can help.

Guys run 600 to 950cfm carbs on stock stroke 340's, but it all in the build and use.