Key is not what the engine is going to cost you intially, but what it is going to cost you to marinize it. What engine was your boat set up for originally, and how much of the marine parts did you get?

If it was set up for the 429-460 and you got all the important parts (bellhousing, motor mounts, exhaust manifolds, etc) and it just becomes a drop in, then you should be ok. If you have to start finding and replacing those parts, or if you are doing an engine swap from one model to another, your costs and the amount of work involved are going to be much much higher.

Tell us what you have to start with, then we can give you better info back. If the engine is as sound as you say, $ 350 isn't out of line. I paid $ 250 for a 460 just to get the block and mains, ended up buying all Cobra Jet parts to beef it up and scrapped the rest.

Don