+ 1 for building a new one, especially if your 327 is still running ok; that way you will still have the car to drive, and can take your time on the build and not feel pressured to get it done, and cut corners. Besides, you are going to have to buy new parts all the way through, and machining will cost the same; you'll have a better and easier result, and still can keep it manageable for the street. I had a '64 roadster with a 365 horse, 327 in it, that was the factory engine with a solid lifter, lumpy cam, 11:1 compression, and high rise manifold with a Holley on it; it was streetable, marginally so, but it was also very touchy, even grouchy (like me), and I had to keep on top of the tune and valve lash, or it would really get cranky. You can definitely get more out of a 327, but like somebody said once upon a time, all it takes is cubic money; in this case the money will be pretty much the same either way - rebuild the 327, or go for a 350/383, and I imagine, in the long run, you'd be much happier with the larger build. Another thing to think about: if your 327 is the original engine, even if the manifold has been replaced, or a new cam, it is restorable. NOS cams, manifold (NOS or used), and other correct parts are available; if you ever wanted to restore the car to original, numbers matching, that is a big step in that direction; just store the 327 for a rainy day. I imagine a correctly restored '64 would see a value gain of at least double over what it would be with the hot rodded stuff in it.

Just some food for thought.