Thanks beenthere. I was reading the fine print last night. If I did that first, I wouldn't have had to ask. Isn't that they way it always is? But it's always reassuring to get a second opinion.
I found I have a maximum of 60' from the popup to the wall cap, to meet mfg. requirements. A 90 elbow that's 3-1/4x10 to 8" round counts for 25'. A 3-1/4x10 90 elbow counts for 15'. A straight 3-1/4x10 to 8" round counts for 7'. An 8" elbow is 7' The wall cap depends on the mfg.
I searched the Internet and just when I was almost at the end
I found what I think is the best option.
They are American Aldes wall caps - (top) heavy gauge G90 galvanized steel, (middle) 314 2B finish stainless steel and (bottom) solid copper type Ca 110. The specs say the damper has a magnet to keep it closed when not in use. Aluminum is $62.89, SS $237.50 and copper $231.75. So aluminum it is.
The design on the above tells me it creates less air drag that the standard design where the top of the outside section is tight against the wall.
I'm redesigning the duct run to change over to 8" round as soon as the 3-1/4x10 passes through the floor. That will create a total of 11' effective length on the fittings. The way I drew it up it would have been 22'. Then there's another 14' of straight run, plus the wall cap. So it will easily clear the 60', even if the wall cap adds a bunch.
And yes, the fine print says to tape the joints. Do I get my tinner's card now?