I put Holley 340 center and outboards on my 383. Mopar Performance reccomended 440 center and 383 outboards for B Blocks, and the only difference I could see between the 440 and 340 centers was the main jets.
I pulled the 61 jets out and installed 63's as reccomended by the six pack tuning tips. Stock '70 383 Magnum, Stock manifolds and 2-1/2" exhaust. Motor runs nicely now. Holleys are about $100 less each than the Mopar Performance carbs, and the 340 center less than the 440. Jets are $6 a pair.

http://www.mopar1.us/6barreltips.html