Just bought a Impala -75 with a frozen 350-engine. It came's oil and water out of two treaded holes in front of engine, just beside fuelpump. I think that they are mountholes for something, so I expect the block is cracked, what do you think?

Could I take a 350-engine from a Chevy Van -88 with injection and changed intake from -75 350-engine? Does it fit?

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the intake on older gen. engines are not the same as the newer gen engines. if you want you can take the newer engine and trans and have new mounts welded in for you, or.... you can just take that older block and have it checked out at a shop.
p.s. some cracks are hard to see with the naked eye because of the rust and old chiped paint. most shops have the proper tools and lighting to see it more clearly. good luck

Now I took the intake off and it was a lot of mixed water/oil there, and four cracks in gallery...

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sorry.... that looks awful. you can search on ebay for some engines in your area. I found me some nice ones on there...