Many reasons this could happen: Cracked block or head, improper block or head prep, warped block or head, too rough of a block for the type of HG used... and on and on.
What type of HG did you install how were the block and head prepped? Was every inspected/measured for straightness, excessive pitting and cavitation and cracking? If the block was cut the rear timing plate MUST be cut to match WHILE BOLTED TO THE BLOCK!
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