Ignition Timing-Where To Start?

Hey All,
Here is the engine:
>New late 90s "Explorer" 5.0
>GT40-"P" heads
>B303 roller cam
>1.7/1 ratio roller rocker arms
>Mallory Magnetic Breakerless Distributor #57 series
>Performer RPM intake
>Autolite 4100 1.12" venturi carb
>Tri-Y headers (currently open)

Here's what I've done:
Engine first started at about TDC, tried 10 degrees BTC at about 800-850 RPM and wouldn't run well or idle evenly under 1000 RPM. It's been suggested that, because the idle won't go low enough to negate the mechanical advance, I should set for "all-in" timing above 3000 RPM. Best running appears to be at about 40 or more degrees which gives me between 25 and 30 degrees at 1000 RPM. Is this in the ballpark? How do I figure out where it should be? Where it is currently seems to run pretty well (when I stand on it, it goes like a raped ape) still won't idle below 950 or so without loading up. With the open headers, I can't tell if there is any spark knock, but engine starts well without "kicking back" on the starter. I've tried setting timing and carb with a vacuum gauge, but don't know if it works well at such a high idle. Maximum vacuum is about 14 inches. Any and all thoughts, comments and suggestions are welcome.
Thanx In Advance,
Gene

D.Hearne,
You are so right and I am so DUH! Engine runs quite differently with a couple of mufflers and some pipe. Thanks for confirming what I though I noticed. Blown65, I'll take that under advisement. You guys have put me on the right track.
Thanks So Much,
Gene