The stock EJ257 block is good for around 450bhp at the flywheel. If you keep below that level you should only need to change Pistons and studs.
Beyond that you'll need to pin the block and swap the rods too.

The block dowels in the pics are about 20mm deep and are an interference fit. The block is slightly machined for them and will need a torque plate hone and a reface afterwards.

There is no reason that a pinned EJ257 built correctly shouldn't make good numbers.

if you're just going to drop Pistons in you must get the block honed to suit the Pistons and to ensure you don't get future gasket problems reface heads and block too.

Spend the money doing it right now rather than several builds later.

Preparing the block is equally as important as the parts you use.