Regarding oil filter boss, the 55 did not have one but the 56 engines did.

On the block, there was a raised ridge along the top just below the head mating surface (on the outside).

There were no motor mount bosses on the sides (common to 283 engines up to 1959)

It has a rope rear seal just like the 283 block up to 1959.

There was no starter mounting holes in the bock (also common to 283 engines until the aluminum powerglide was introduced)

The cam had the flat for oiling on the rear bearing

There is zero performance advantage in using a 265 block. Zero, regardless of what you do. They have only restorer value. Most have been melted down to make nodular iron water pipe....like Tyler Pipe in Tyler Texas...they use engine blocks and heads because of the quality of the raw metal.