Frustrating to say the least..........

Got the arm rehabbed enought to remove the radiator and took harmonic balancer off and removed the sling ring that was spinning and chattering......
Now she purrs like a kitten.........FINALLY.
But found out it is not the original engine.....its a 1967 273 block with hydraulic lifters.
So since its not original anyway.....I was thinking:
I have a line on a 318 block with pistons still in FOR FREE the best, no heads intake or oil pan that was pulled out of a car with casting numbers as follows:
2536030-318-8
casting date 8-13-71

I'm thinking of building the 318 and swapping it into the wagon at some point...in the opinion of the forum would it help or hurt value to this car??? the 273 runs good and smooth right now just thinking of the build motor for something with around 400 hp? I like the 318 build threads on here and the magazine article links but does Anybody have a good cost estimate for a 318 build with that kind of hp from the build talked about here ? Who has ACTUALLY done one of the builds and can give me a good estimate.