Crank bolt to hold damper

There's a good thought! The bobweight that I have for the 5.9L is around 2004 grams. That is quite a bit different than the LA stock bobweight that is up around 2150 grams.

But we don't know what your engine has inside for rods and pistons. Do you have a balance sheet from the build?

The LA damper and flexplate/TC are made to work with that heavier 360 LA bobweight to reach balance. The lighter 5.9L bobwieght would work with a different external weight. I just looked up the B&M flexplate for the 360 LA and, sho'nuff, it is only for the LA, up through 1992. All I can find is that the Magnum external weight is different. 5.9 Magnum to replace Older 360, flexplate?

And the stock damper weights are different too from 360 LA to 5.9L Magnum. Professional products balancer: which weight for magnum

So it very much looks like you would need to re-balance. But, if you have bobweight info from a balance sheet for the build, then you only need to balance the crank end external parts with that bobweight. I bet you can modify the external LA parts to have the lower external weight used for the Magnum; you would be taking off weight from those parts.