I have never done what you are proposing so there is no experience here, but I would use all
original holes. Those engineers weren't stupid
and they got away with as little as they possibly could already to save production costs.
I haven't studied every street rod I've come across in depth but as I recall every one I have looked at in the motor mount area used all the mounting bolt holes in whatever block was being used. If you don't do it and ruin a block
you will be kicking yourself in the butt for quite some time I bet.
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