Might want to check the spark plugs to see if the air/fuel ratio is right. Should be a light brown color.
If the cam was changed from stock & is a bigger aftermarket cam or if the previous owner slapped a carb in place of the EFI without jetting the carb, the carb's primary jets might be too small and causing a lean mixture until the huge quadrajet secondaries are kicked in at WOT.
The 650 CFM carb should be fine with the L98.
Use to have a '69 Camaro SS/RS 396/M-20/3:55 with a mild aftermarket cam and GM HEI. Cam was advanced 2 degrees to allow for chain stretch.
I used a Holley 650 CFM Spreadbore double pumper with mechanical secondaries and the engine ran extremely well with the 650 so the 350 CI should be fine.
Changed the vacuum advance unit on the distributor to a Moroso adjustable vacuum advance unit as well as changing the centifugal advance weight springs helped to dial-in the advance curve with 12 degrees initial and 36 degrees total.
The adjustable vacuum advance unit helped to bring down the HEI's advance curve which introduced too much advance too soon.
I also connected the distributor's vacuum line to the carb's delayed vacuum port.

Checking the engine's vacuum with a vacuum gauge is a good idea.
If the vacuum is extremely low, it could be from an improperly installed intake gasket which I would first check the tightness of all intake bolts....an indicator is oil on your plugs as the oil is sucked from the underside of the intake manifold. Another source is the carb's throttle shaft which might be worn. To check put some oil by the throttle shaft to see if it gets sucked into the intake or if the primary throttle shaft can be wobbled in its bore. Check all rubber vacuum lines for leaks. A hissing sound from your power brake booster is a leaking diaphram & will drop your engne vacuum & cause the engine to stumble off idle which would require replacing the power brake booster.