Channel balance
This is an old rod type balance that isn't made anymore. Can you first tell me the diameter of the spring? If it's 1/2", then you can use our Series 795 as a substitute. I recommend watching the video below, which covers the conversion process step by step in detail.
I'm sorry to hear that you're having trouble. I have actually heard of this issue before, though it's very uncommon. There was once an older version of the 26-366 that was longer, and it sounds like that's what you need.
The bad news is that we don't carry anything that works as a 1:1 replacement for that old pivot bar. I haven't dealt with these often, but when I do I opt to drill a new installation hole so that it can scoot forward and make up the difference in size. I know that's a bit of work, but it should fix things.