What engine can I have?

I wanted to participate in the car building experience. I recently bought a 1966 Dart 170 4-door. I chose this car just because I could offend noone if I messed it up. Engine was knocking and I needed several parts. I bought a 1966 270 with a horrible body and stripped it. I used the interior, windshield, motor already. I plan to install the power steering, auto tranny, and A/C this year. The motor in the 170 was smaller than this 225. Are they the same block with a different head? If so is it worth it to rebuild it as a 225 or should I just get a V-8? If V-8, which one was available in this year car? I am only putting on parts that would have been available that year. Thanks for any help or advice.
Chris