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The 235 radiator will work fine.
A 1954 Shop Manual has information on the 1954 261 engine.
You will not find a GM Shop Manual with information on how to install the 261 in a small truck. This site is your best bet for that information.
Go ahead and ask the questions. You might want to do a search first (like "261 radiator", in this case) but you will get answers even if you just ask questions.
As everyone above has said, this might be new and strange but it is not technically difficult AND you will indeed like the extra power from the 261.
Tim
