Not really sure about your first question. The blow down is the difference between the exhaust port timing and the transfer port timing. Most pipe design programs I've seen use the actual timing figures, not the blow down number.

Most non pipe engines do best with exhaust timing of 160 or less, and pipe motors 160 or more. It depends on the rpm range you want. I would probably start at 170 degrees exhaust timing. You probably already know this, but it's best to make the pipe length adjustable to fine tune it to the engine.

AV8TOR