No way is that 750 carb size too small for a street 350, more than plenty.
I raced a moderately-built 455 with a Holley 650 double-pumper on it for 10 years, ran 12-seconds flat in the 1/4 mile with it. Tried a 750 DP and it didn't run any quicker, I was running a cam that's much larger than the one you have (221/229 duration @.050 for the 60916, 244/252 for the HC-03 I ran).
Your carb is big enough, but I'd replace it with either a Holley or a Q-jet if it was my car.
