I"m not an expert on carb sizing. I only know what I've run on various cars for the last 40 years or so. A 750 may be a bit large for a 390 with some decent power, but it's not out of the ballpark and I wouldn't throw it away (or sell it) to buy a smaller one. Tuned, it will work fine. I ran a 750 DP, manual secondaries on my GTO (400 CID). The original Quadrajet was 750 CFM, vacuum secondaries. I ran a 3310, 750 CFM, vacuum secondaires. I even tried a Holley 3-bbl, 950 CFM. Anyone remember them? The best performer with the best response was the 750DP. The best mileage/performance combination was the 3310, but the Q-jet was close. The 3-bbl was the biggest PITA to tune, but it ran as good as the 3310 once I got it there.

Shoot, the original 302 Z-28 even ran an 850 CFM Holley clone.

Just saying that I'd really work on tuning the 750DP before I spent a lot of extra cash on something else.