66 valiant 170

Hey guys. Definitely enjoying leaning on the 66 valiant. I was able to decide the fender tag and vin and figured out I have a 170 with. A 903 three on the tree transmission. How do I tell if I have an original motor to the car? I can see the casting number and it goes a 1965 170 and the tab behind the alternator says 170. I’m wondering if my uncle swapped motors and out a 65 slant six in there or if it’s normal to have 65 motor in a 66 car (started building in 65 kind of deal). Hoping to do a master cylinder swap and replace all the lines then start doing some body and paint work. Considering putting a 225 in it, sounds like it will hook up and if I get a post 67 motor I’ll just need to swap out the fly wheel, oil pan and pick up ( possibly add a pilot bearing). I’d like to rebuild the 225 and hang onto the 170 because it’s a cool motor. If I missed anything or anyone has opinions feel free to chime in. I appreciate everyone’s feedback and look forward to learning more about the car.