And Fidanza makes a 11.5lbs flywheel for your application, I'm sure.
P/N 43301 . I just bought it but do your research. It's cheaper than a kaemari. You can also take a look at JGY Customs.
Take a look at SPEC's website and do some part number checks. If the 83 ZXT and the 87 Turbo or 85 Turbo flywheels are all the same, then just buy one from an 85 Turbo, which would be 240mm. The FS5R90A was a T5 transmission as well.
Listen to your father. There is no "need" for a flywheel assembly weight. Have the unit balanced when your rotating assembly is balanced, and it will be good.
being a 3 litre, I've been told the 11lbs flywheel should be easy to drive because of torque, and you're making a stroker here = more umph.
But really, I'd wager that 15lbs is a good spot. Not too light, not too heavy.