Try this long read that I saved..
Here's some gold for you from Gary Penn of GM Performance Parts. I have several of his posts saved from a techboard somewhere on the net - I think it might be chevytalk, or chevelletalk or something like that.
Here is one of his posts that directly answer’s some of your queries. BTW - if you use Vortec heads you'll be flat amazed at how much power these things make for a truck head. I run them on a 413-cube small block and have nothing but positive comments for them.
Let's proceed directly to Gary's very detailed and insightful comments:
"Two years ago we undertook an exhaustive (no pun intended) study of the Vortec head in numerous modified states with different valve sizes, throat cuts, valve jobs, port mods, guide mods, etc. We used up about 10 heads, numerous valves, and about $50,000 worth of labor. The study generated 100+ pages of flow and swirl data, which I have at my desk.
Here is the short version. Out of the box, .480” valve lift, 350-400 HP dependant on the CR, cam, ring seal, oil control, blah, blah, blah.
More than .480” lift cut the guides down to clear the retainers.
Straight mill up to .060” safely, .080” with low CR of 10 to 1 or less. 0.100” is living on borrowed time. Always use flat top or dished pistons to enhance flame travel and intake swirl.
Angle mill up to 1 degree (about .110” off the exhaust side, .000” off intake side) safely for about 12 to 1 CR with flat pistons with little valve relief. Angle mill to 2 degrees (about .200” off exhaust side) if you like to live on the edge, it has been done.
Larger valves increase flow, chamber mods not needed, trade off between shrouded verses unshrouded valves not worth the decrease in laminar flow and swirl.
Throat cutting behind larger valves compliments the larger valves. Open the throat to the seat then remove the edge left by the cutter in the port.
Blend the seats into the chamber, you don't want an edge here to disrupt flow and create turbulence.
"Bowl blend" and shortened guide in port also improves flow. Taper and blend the iron boss.
Minumal porting increases flow, too much increase in port size or loss of the benefits of the shape of the stock port will decrease efficiency.
Vortec heads (and most others) like straight stemmed valves. Undercut valves create unwanted turbulence and a decrease in intake charge velocity (they add volume (slowing the gases) to the overall "port" volume just behind the valve where max velocity is required.)
Generally speaking, Vortec's stall at between .500” and .550” valve lift. This is where flow actually begins to decrease. But their true strength is low lift flow which gives more area under the total flow curve. And if you think about it how long are your valves at peak lift? They spend much more time at .400” and below, where the Vortec’s outperform most other heads. This combined with high velocity, lack of turbulence and superior combustion chamber design are where the Vortec’s stand out.
Unported, with all the other tricks in place, the Vortec’s will flow about 235-240 CFM at .500” I and 165-170 cfm at .500” E, on a 4" bore at 28" H2O, with clay radiused port opening. With some careful porting there is another 5-10 CFM or so to be had. But again the low lift numbers are unsurpassed at .100”, .200”, .300”, etc. lift. For example the Vortec’s flow as much air at .400” as .500” and no 23 degree head that I'm aware of can match them at .200”-.300” lift for the combination of flow and swirl. Even the Fast Burn head can't touch them at low lift, it's ports are too big (flow is similar, swirl is less), it does of course out perform them at lift over .500”.
Unported Vortec’s with the "tricks" can produce 500 HP on well built, high CR, drag race short block. 425-450 HP is more realistic for a killer street engine running on pump gas."
Per Dirt R. You will love the Vortecs on a 400. Incredible torque and overall power. I bowl ported mine, and did some minor work in the runners. At a minimum, fully bowl work the exhausts, as they are the only real weak point on the Vortecs. You can use the stock intake and exhaust valves (which are back-cut from the factory) and the stock first-rate factory 3-angle valve grind. I came within a whisker of an 11-second pass in the heat of July running Vortecs on drag radials with a cheap Crane Blazer 234/244 hyd. cam. I ground off 3/16" off the bottoms of the factory retainers for retainer to guide clearance, and used Crane stock diameter valve springs.