Ive done that before. Its from the engine slapping the chassis. If its 14gal old then its most likely done plenty of slapping out on the chassis over time. Thats a very thin part of the block.
At FEMCA last year they had a log section that you'd slap the chassis on if you got it wrong. Cost me an RB C6USA Rody.
If your really worried about it happening again just put some washers between your mounts and the engine or buy the m2c engine spacers