Something you all need to consider on this "choice" issue between the pension and 401K. While it is certainly something most in the union membership would desire, pensions like insurance are about risks, investment returns and actuarial estimates.
My view is that if I was near retirement, ceteris paribus (there's a $5 phrase for everyone) in terms of company contributions between 401K and pension, I would choose the pension. For example, keeping the math simple $80(?)/month per year of service, so one year equals $960 annually. Safely I would need to have around $10,000 in retirement to equal that benefit over a lifetime to draw down against, but the company has been contributing $1.05/hour to the pension which is only about $2,200 a year for full time? So $960 return on a $2,200 investment-- that's a nice return on investment! In contrast, if one has 30 years to retirement then the magic of compounding interest (even just looking at what the company contributes) creates a sizable 401K which returns a larger monthly income than what a $2,400 ($80 x 30) monthly pension would provide.
My point being that if given the choice, I suspect those close to retirement would flood into the pension and those with decades to retirement would take the 401K option instead. It would leave the pension in even worse financial condition. Like health insurance, pensions attempt to pool the risk between expensive and less expensive policy holders/beneficiaries to average out the risks. I don't think it is a viable option for the IAMNPF to allow high return retirees (those close to retirement) to stay in the pension while the low return retirees (those decades from retirement) to opt out.
We are seeing the same thing with ObamaCare state health insurance exchanges where younger, healthier people are ignoring the personal mandate and paying the fine, as it is cheaper than the premiums. However, older, higher risk, less healthy people are willing to pay the premiums, especially without penalty for pre-existing conditions, and the results are predictable as insurers withdraw from the market.