Well, that was the caveat…the block needs to be in good shape.  A lot of 428 blocks are susceptible to cracks.  Gotta be diligent.

I've done some 600+ lower rpm stroked 390/428's like that without needing crossbolt mains, etc.  I feel comfortable with 600 hp on a factory block without extra stuff done.

Inlarging the PVCR's from .040 to .070 allows sufficient fuel were the previous lean condition was, allowing the main jets to be properly tuned instead of causing a pig rich condition covering the lean condition caused by the to small PVCR's in the circled in the power valve mount.

The 2 holes circled to the left, the hole to the right feeds the transition slot in the throttle plate and the channel the hole sits in at the top connects to the main well. The hole to the left is for the air fuel mixture and feeds a hole in the throttle plate below the transition slot and the butterfly. The butterly needs to cover the majority of the transition slots in order for the air fuel adjusters to work, usually about .020" is uncovered.

To set the idle and to keep the transition slot mostly covered so the air fuel mixture screws work, some times the 2ndary is open slightly to increase idle, or holes are drilled in the primary butterflys to allow air to flow to increase idle allowing the air fuel mixture screws to work.

For fine tuning, sometimes the idle fuel restrictor which is to the right of the 2 holes circled is removed and relocated to the top of the channel from the right hole in the transition circuit that connects to the main well at the top, the connection to the main well is taped for adjustable idle fuel restrictors for fine tuning, which may or may not be necessary, just because it can be done doesn't mean it needs too. 

Once the primary has been sorted and the throttle operation is smooth and trouble free, then the 2ndry tuning can start

Thanks Gerry, Ive fought the 4 bolt for 30 years and think I'll take an easier route this time, Mike I hear you on the 4 bolt and OCD as I've been doing it that way since the beginning , Fastback 427 I been using the small bolt headed bolts too but my Crites headers are still tight

Thanks for posting this i have a few 3 barrels i always wanted to try out. I remember Barry R doing some dyno work years ago and the 3 barrel beat them all except a dominator. So just drilling the hole from .040 to .070 is the first step?

Thats correct .040 to .070. 2 holes under where the power valve screws in. The Dominators replaced the 3 Barrels I believe it was 69, thats why no further development was done on the 3 Barrels. And  Holley has the 1050 @ actually 1030 cfm.

I remember that Dyno run, I think I remember one of the other carbs being an HP 950.

Here's the link.
https://www.youtube.com/watch?v=F38HumC41uY