Originally Posted By A/MP
... take a stock 1971 340 cam spec 462/473 with a twist of 320-330* of duration. How do I calculate how much more lift can I substitute and lower the duration and achieve the same results.

You can't.

Google "David Vizard cam duration vs lift" and see what you can find. Although not a direct comparison like you're looking for, he does have at least one example showing how a lower-lift/longer duration cam performs vs a high-lift/shorter duration cam with both making about the same peak HP.