[wkooiman]

2 Holley 600 vacuum secondary carburetors are not too much. The secondaries only open if they are needed, so it's basically the same as a 600 mechanical secondary with an extra 600cfm if you need it.

That's assuming you have the proper BJ/BK, BC/BD, AC/AD, etc. carburetors and progressive linkage. Ford spent quite a bit of time getting these right. They weren't too much carburetor in the 60's, so why would they be now?

If you aren't using a stock style setup, all bets are off.

I previously ran off-the-shelf 1850s (600cfm) on a 428 that were reworked by QFT. They ran like a dream.

BTW, if you google this subject, you will find several threads. In one thread, someone had lots of problems with 2 390cfm carburetors. Stick with what works, and you'll be happier.