I personally love the 400. Huge bore, ready for a lot of extra cubes should one decide to change the crank, and still a Low Deck motor for better fitment. Again, I love the 383 too....
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If you have the money to build it the 400 would be my choice over the 383. However the 383 will walk the dog on the 400 in stock forum. And by this I mean a 68 or 69 383 versus a 72-73 400.