The 389 heads used shorter pushrods, under 9" long. The 455 short block may have a cam in that has more lobe lift than the valve train can accomodate, & is bending pushrods. Check that the the springs are not coil binding, the retainers are not hitting the seals/guides. Do the 455 pistons have the valve flycuts in the correct location for the 389 heads?
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