If you're careful. The right rods help a lot.
Some blocks, pre 1970, require much more work with a hand grinder. Pre 70 blocks are not notched to even clear a 4.00 crank, so you really have your work cut out for you regardless of the rods. Notch the bottom of the cylinders too much, and you're into the water jacket. Grind too much, if you need to(probably will) on the left side of the oil pan rail, and you're into the main oil gallery
The crank will have to be one cut for 6.535 rods. A crank with counterweight for a 6.7 rod like a Callies will run into big issues with the counterweights.
Yes, if can be done. The question is, WHY? You powering a tug boat?
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