If you take 2 carb's:

390 cfm 2 bbl
390 cfm 4 bbl

You will get better throttle response from the 4 bbl than the 2. Its all about booster signal - smaller venturi - high air speed better signal & better driveability.

If you get a 390 4bll vac sec. and tune the sec. opening rate it'll work well.

51 GMC 4.2 turbo
Can't solved today's problems using the same technology/thinking that created them