You say its a '55 261 ... There were two series of trucks that year -- the last of the Advance Design and the first of the Task Force. The difference in the engines is significant in this case. Give us the casting number on your block. If the engine was built for the '55 1st series trucks, it will need far less modification to fit in place of a 216 (practically none -- if you use all your 216's peripherals. Use the 216 flywheel if you want to keep your 6 volt starter and the oh-so-cool foot starter pedal ... and who wouldn't??
). If it is for the '55 Second Series (Task Force), then yes, you will have more work to do to make it fit.
I replaced a 216 in my '49 with an early 261 and had to make zero mods to make it fit. I used all peripherals (swapped out the timing gear backing plate to get the correct front motor mount), except the manifolds -- need the bigger Rochester carb to feed the bigger engine ... The good ole Carter W-1 is a great carburetor but it's nada bueno for the 261. I even used the 216's water pump without modification to either the block or the radiator. No need for electric fans, moving radiators, installing adapter plates, blah, blah, blah ... *if* you have the early series 261.
Don't do anything crazy until we figure out which block that is -- it could potentially save you a lot of time, effort and $$$
Here's my 261 installed. I used the 216 valve cover, too. Which, considering all the oil everywhere, may not have been the best decision...
