Hi Patrick!
Welcome to the Imperial Club!! I would keep and rebuild the 413 also. Not for originality's sake at all, but performance. Just to make a couple of hands-on comparisons, my 413 in my 1960 Crown 4dr walked all over my 72 Newport that had a 440.....granted it was a smog motor, but still....and my Imperial is no spring chicken either (both it and the motor have 100K+ miles). The 413 in my car performs just as well as a souped up 383 I had in a road-runner, and that car was 1200 lbs lighter than the Imperial.
The 413 has the "torque" feeling of a 440, and the "horsepower" pull of a beefed 383. This is because its bore to stroke ratio. I'm sure that most of the Imperialists would agree that the 413 is one of the better of the big block Mopar engines in stock form next to the 426 Max Wedge or Hemi. Unfortunately, the 400---to my personal experience---has been a "smog-dog". Only 1 out of the 5 that I owned actually had some power. The rest I could not even keep running properly, and I'm a performance nut.
Anyway, I've rattled on enough.....hope this helps. Again, welcome to the Imperial Club!!
Chris 60 Crown 4dr, 57 Plymouth "Christine" to be, 57 Dodge 2dr HT, and Chargers!!