Some parts from an Impala are interchangeable with the Catalina, but not much - cosmetic only tho. Mechanically, the Impala and Catalina are two different beasts.
Your best bet is to locate a service manual for the Catalina (1963 with 1964 supplement) and start there.
First, before you buy the car, make sure it's complete especially the side trim moldings (and make sure they are useable) as those are extremely hard to find and very costly if you do find a set (like $1500 for a decent set of the 6 pieces)
Check the rear trunk body mounts (PY has them) and frame rot under the doors is the first thing you need to check.
This is the right place to post your questions for you '64 Catalina - best forum for Pontiacs on the web.
It would likely be a 389cid engine, 2bbl carb, with the roto-hydromatic transmission (a.k.a. "slim jim"), which is a POS - but can easily be replaced by a TH400.
Performance Years has a great list of reproduction parts for the car, but some you will have to acquire via "good used" (
www.indianapontiacs.com)
Ron