The 6X heads are great heads for a street 455 combo but you will end up with very low compression on a 389 due to the smaller bore and shorter stroke. To achieve even 9.0:1 you will need to mill the crap out of them. Then you will also have to mill the intake side of the heads as well to assure proper fit and may even loose proper intake manifold alignment in the process. Also milling the heads that much may cause piston to valve contact. I think another head choice may be an easier road.
Tim C
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Tim Corcoran
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