Thats really weird that the block is that bad but you managed to unbolt everything else and you still have useable parts, especially the crank. Did you get a third opinion from other shops? The reason I ask Is most of my experience with my local shops here are that they can sometimes be extremely picky and just want to do big paying jobs such as race motors for sponsored customers. If you show up with a motor and you just want it cleaned they may try and talk to out of it or they will take your job but if someone shows up with a huge paying job they put yours on the back burner. Have you tried pouring a concoction of rust removers in the cylinders and just let it sit over night?
BTW $100 for a big block isn't so bad! Even if you can just get everything freed up,cleaned then sell it to someone who does want to put money into it. Also the table idea works too.
Just make sure you check over your new motor thoroughly. Just because it looks super clean and he had a nice speech ready for you doesn't mean anything. The fact that you got a 400 and a 383 for just $250 is awesome, your ahead of the game. It's no set back, you have extra parts now that you can potentially profit from.
