It won't be a 1 to 1 integration.
JetBlue would start below the widebody FO's, not by who holds it, but by total seats available.
So say JetBlue starts at 3,000 or 4,000, and then would integrate with the 737's and A320's. The actual question ends up being whether the A321 is equal to a 757, and that would be up for debate.
The three items that ALPA merger policy looks at at longevity, career expectations, and category and status.
There would be fences around widebodies for a few years, but nobody is doing long fences after the disaster that redbook/greenbook was.