I just replaced a 216 in my '53 1/2 ton with a 261 from a '57 model 2-ton. Hard to tell from that picture if you have a 235 or 261 there, but the engine code by the fuel pump and the cylinder head code by the manifolds will tell you what block/head you have. A few other members should weigh in soon about how to "wake up" an engine that has been sitting for a period of time, and how to properly check everything before you ever hit the starter.
(Editor's Note: Members *have* weighed in on that and
we made a Tech Tip out of it Happy reading)
A few notes from my swap (I had my 261 rebuilt by an engine builder):
1. I used the water pump adaptor: https://www.ebay.com/itm/1955-1962-CHEVROLET-235-261-WATER-PUMP-ADAPTER/142578007830?hash=item21324fbf16:g:jz4AAOSwmBhaCNA7
2. I used the 216 bell housing and foot-starter hardware
3. The front of the 261 block had to be drilled for the front 216-style motor mount. Make sure to do this before you put the engine in the truck.
4. You'll need the starter from the 261, so I would have it tested and if necessary, rebuilt.
5. Electronic ignition is pretty nice to have. A fellow stovebolt member helped me with a 1980's 250 HEI distributor collar modification that allows it to work in a 261. I found a re-manufactured OEM unit on RockAuto for about $55, and bought the top portion of the distributor (cap, wires, coil etc) from Napa. The "aftermarket" units made in China appear to be cheaply made and unreliable.
6. Make sure you have the correct carb for that engine, whether its a 235 or 261. You can find measurements on this site that will help you confirm which carb you have and/or need. From your picture, it looks like the engine has a Rochester B. In my opinion and experience, those Rochesters suck. The 2 that I own are going to be thrown off of a bridge after I put my Zenith 228 on soon. From what I hear, a Carter YF or Zenith 228 is a great option for a 235 or 261, just get the right model number that was meant for that particular engine.
7. If you do end up rebuilding this 235/261, consult with a few of the members here about must-do items before you tear into it. There are some very helpful tips that will save you headaches later (such as using a rubber or even modify the block to accept a 1-piece rear main seal, instead of using the old 2-piece rope-style seal that could lead to a major oil leak. Don't ask me how I know about this issue...
