Advanced Engineering Tech - How does altitude change header pipe diameter needs?

I'm putting an LS-1 in an RX-7. There are two headers that fit and one has 1 3/4 primaries and the other has 1 7/8 primaries. I know that as altitude increases, it has an effect but is it bigger? Smaller with higher is my guess but I'm far from sure. The particulars for my application follow; Stock 346 with a small cam (TR 224), with headers for starting but later to add better heads and intake. Any help is appreciated. Oh yea, I live at about 5000ft.
Stan

think about breathing through a straw at sea level, then go up to the mountains and try the same thing its even harder on you. Same goes here you are working with thinner air up there so you need more flow to make the same HP I will make here at sea level.

Go with the 1 3/4"

With less dense air the engine breathes less lb/hr of air.

At your altitude you are effectively loseing about 20% of your HP. The 1 3/4" primaries are more than enough to support your HP output.