FYI, SEMA's page only publishes info they get directly from each state's DOT, so it can be somewhat biased IMHO, but it is a pretty decent overview.
According to that info, Hawaii apparently considers your car to be a "reconstructed vehicle", since it was modified "by the removal, addition, alteration, or substitution of other than original replacement essential parts, including the vehicle's... ...bumper system...". This would justify their denial.
Curiously, bumpers are considered "optional equipment" on street rod vehicles and street rod replica vehicles and pre-'69 vehicles. (So much for consistency.)
It may be simpler to buy and install some "original replacement" bumpers on your car, and be done with it. (I'm assuming the aftermarket ones are not DOT approved.)
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