Given the vintage of your tandem, I would speculate that it originally did NOT have 145mm spacing and was probably cold set at some point during it's life.
Given this, I would not mess with attempting to cold set it again.
I have messed with washers as shims but have always found that you have to futz a lot when mounting the wheel as they never want to sit tight to the outer locknut (ie futzing and greasy fingers). This would only be a consideration depending on your circumstances - do you remove your rear wheel often (e.g. to transport the bike) or just occasionally for maintenance?
I recommend you find a new wheel and get it back on the road.