I think Panic "hit" on part of the tuning thing - changes in x-section area. "Open headers" really just means the x-section area ratio becomes infinity (or zero). But if you had a huge muffler, say a 55-gallon drum, the exhaust would "see" pretty much the same thing.

Often a dyno room will just use huge sewer pipes (6" diam or so) to route exhaust out of the room - hoping the effect is the same as open headers. Its not, but its pretty close.

There ARE formulas around for pipe length vs tuning RPM, but they always depend on valve opening / closing. Measured HOW? Seat-to-seat? I usually don't like the answers I get from those.

Usually you end up with primary tubes determined by the fit in the car, unequal length and to me that's fine for street or drag since each cylinder will have a different "peak HP" rpm.

The collector length (including ideally the exhaust pipe that's the same diameter) then gives a lower RPM peak, say the peak torq RPM. This length we can tune, but that has long been done at the strip using open headers, literally cut & try to get the best ET's. But its not too hard to do. And of course in any tuning, you do have the compromise that the pipe DIAMETER has to be big enough to avoid backpressure, which is always bad, but not so big that you lose velocity and the pulse signal, at that point your tuning effect is all done.

Then, that's where you want the 55-gallon drum muffler.
Of course that's usually right under the floor pan, the worst possible place for ground clearance, let alone a 55-gallon drum.

So a compromise is to put an H-pipe or X-pipe at that spot, to fake the exhaust into thinking that's "open" to the air.

Of course it wont be fooled so you will get some pulse effect but not all of it.

From there, the best you can do is put the biggest mufflers you can fit as close up as you can, typically under the back seat.

After that, the pipe size doesn't matter so much, you could use a 3" or a 20" pipe or even a smaller eg 2.5" because the exhaust has cooled somewhat - you just need enough pipe to get the exhaust out without backpressure.

This is kinda what the factory tended to do, they just made more compromises for cost than we can.

Remember, above all, the six was meant to be CHEEEEP - not just good on gas, but cheep to make, cheep to repair, cheep to own.