Engine swap 305 short block with OMC 800

I have recently purchased an 1978 18'Thundercraft in great shape. However, I purchased the boat knowing that the previous owner had the engine inspected and water was in Piston #5. Further investigation showed there was a crack in the block. That being said. My question is such; Can I do a straight swap of parts from the original block on to a identical block that came out of a 1980 GMC Pick up? I have been informed that I need to switch the cam over but other than that is it a fairly straight forward procedure? Please let me know as its becoming hotter that F&*K here and would love to get out on the water. Also I have a 350 short block if that's an easier swap from the 305.

Thanks

Craig