That doesn't sound right. I have never heard of that although I'm not saying its not the case. I think if it is not cut for a key that you might run risk of snapping off the end of the crank or damaging it.
the lt1 motors have that setup,the crank and balancer are made for each other.
the keyway is 5 inches down the crank. im pressing a puuly on that wont adjust a hair once its on a tiny a bit.
whats the odds of lining up a keyway?"it dont have one damper anyway"
Do regular balancers just slide on and the keyway and bolt go on?
With the power you're making, I would figure a way to not run without a keyway'd set-up. You are correct about the LT1 motors, but they get concerned with power and the lack of a keyed assembly, even when the keyed assembly is missing by design.
If there is some sort of marking on the balancer where a keyway would be (or is) and you know where the key is on the crank, you could get close by making markings on the outside of the balancer for reference.
Otherwise, get it to TDC on compression and install the damper straight up. Even this wont be exact, but it will get you close. I really would look into gettin a new damper or this one machined for keyed set-up.