The first thing to check should be your VIN #. It can be found on the door plate (also so some other places around the body). It should look something like this: 3R01U108543. The fifth digit is the engine code; S is a 144 & U is a 170. If you still have the original engine. Another way to make sure is to look on the front left (driver's side) of the block. There will most likely be a tube coming out of the uppse part of the block, near the head. Just above where the tube comes out there should be a flat spot about 1" by 1/2". Take a wire bush to this area & clean it off as good as you can. You'll see that the spot is divided in half, one half should be machined flat, the other is rough. There will be a series of numbers & letters stamped on the machined area. Supposedly these are an inspector's stamp, but I'm not all that sure. In the rough half there will be a cast-on letter. It will be an N or a T. N is a 144 & T is a 170. If you're not sure if you are looking at the right spot I can take some digital pics & show you. Hope this helps. Take care,
Edwin
ps. your casting #s are the same as my '61 170 spare that I've got. Very possible you've got one of these early 170s.