Pete C. wrote:

I need help in selecting an extension spring to counterbalance most of a
3.5# load over a hair less than 1" of travel. The 3.5# load needs to
rest solidly against the bottom stop at rest, and I need most of the
3.5# load counterbalanced through the 1" stroke, so it seems I need a
spring where the rate will not change significantly over that 1" travel.
I have a good amount of room to mount a fairly long spring, up to about
5" contracted length.

I see plenty of springs on McMaster to choose from, but I'd like to be
able to order the correct spring the first time.

Thanks!

Why not call a spring company and speak to somebody who knows springs?

There's one here called Stanley Spring and Stamping Corp.