If the engine is really a '56, then it would be a bypass system, and the oil lines would be 1/8" NPT at the block. On '58 and earlier 261's, and all 235's, the rear oil line goes to the filter, and the return line from the filter goes to the fitting near the center of the block.
'59-63 261's are as Titen says. they are 1/2" NPT at the block.
