I have an engine that I have diassembled. Parts were on the garage floor in boxes when the river decided to flood over its banks. Question is can this block and heads still be used. The parts stayed under water for a period of approximately 20 days. It is a gm 454 out of a 74 1500 pickup. Its already been bored 30 over. Is there any way to get the rust out.
Start with an oily rag to remove as much as possible from the cylinders. If the bores haven't been honed yet, that procedure will take care of the remaining rust. If it has been honed already, after the oily rag, wash the cylinders with brake parts cleaner or carburetor cleaner to remove the oil, then hone them lightly with no lubricant. Any oil or liquids will make the rust plug up the honing stones. Don't get carried away because you don't want to increase the diameter of the bores. If necessary, it's better to leave a little light surface rust remain.
Use an air tool with a Scotch pad to clean up the deck surfaces. Since they're cast iron, there's not much chance you'll make a low spot where the head gaskets won't seal. Surface prep isn't as critical for the gasket to bite into as it is with aluminum heads. Same for the heads. Scuff the gasket surfaces just enough to remove the rust. I wouldn't panic about any rust in the intake runners. If that offends you, use a porting kit to clean them up.
The biggest concern is the crankshaft journals. There may be almost no rust on them if you dried them off as soon as it was removed from the water. If a machine shop ground them, they would have coated them in oil and that might have saved them too. If they are real rusty, have them ground to the next standard undersize and use matching bearings. For very light rust, you may be able to clean them up with fine emery cloth. Run a valve guide brush through the oil passages and clean them out really well. The biggest chance of a failure comes from debris scoring the main and rod bearings.
Clean the camshaft journals and lobes too, but I'd be real hesitant to use emery cloth on the lobes. Spend as much time as necessary with just an oily rag. If the rust is deep and has caused pitting, just replace it.