I see that the maximum current draw on the Arduino Mega is 40ma per pin.
I plan on creating a simple LED project using all 70 pins. Most the time they will not be on at the same time but I do hope to have all 70 on at the same time periodically. Each LED is 20ma and well within the range for each pin but if all 70 are on the current for the whole board would be 1.4A. I think it can likely handle this but I'm just wondering if someone actually know what its limits are?
This has been discussed before and there are several current limits that apply of which hitting any of them is the actual limit you have to live with. There is max pin limit that you listed, max port (group of 8 pins) limits and total chip current limit. The total chip current limit is determined by the number of ground and Vcc pins that the specific chip has as each of the power pins (ground and Vcc pins) is limited to 200 ma. I believe the total chip current limit for the mega2560 chip is 800 ma.