externally, 53-62 arms are the same, you just need to make sure that if the block is a 53-57, that it uses the arms with the annular groove in the center of the rocker. 58-62 blocks would use the arms with the groove offset slightly to one side.
Many people have used the 235 springs on 261's without issues, but obviously, the 261 springs would be better. Dual springs from Clifford would be better still, especially if you are using a performance camshaft.
