So you've got a 20 year old American pushrod V6 with nearly 170,000 miles that you've pumped up to ~330HP with an original, stock bottom end. Needless to say, it is going to burn oil, especially driving it the way you are.
To answer your original question about fresh oil burning faster... it's common for some oils to evaporate or "burn off" the lighter components when it's fresh, until the oil level eventually becomes more steady. This does not happen with all oils, and it depends on other factors including the type/condition of engine and driving habits (hot rodding / short trips), etc. If the oil level continues to drop significantly after a few thousand miles, either you need to switch the oil brand or viscosity (which you have been doing) or you need to concede that your engine is an oil burner. The good news is that you're not burning a ton of oil and your engine may live a very long time since you're keeping it topped up. I agree 10w40 is a good choice for this motor. Good luck.