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I love a big block in anything you can squeeze one in. BUT...for ease of swap, 350, not to mention gas mileage and longevity of your th350.
350 with a big cam headers intake and msd will do you good.
I think a rod exited the 400 block while the engine was running.
Why not just rebuild your 400?If your 400 hasn't already been bored, I'll sure will take it off your hands.