Piston pin to rod clearance

First, you shouldn't 'drill' the rods for pin clearance, you hone them.

But why bother with a low rpm cast piston? Although those pistons accept circlips for floating pins, there is no benefit for a low rpm street engine.

Depending on the rod, bushing it might weaken it with the metal removed for the bush.

Expensive operation for no benefit. Probably cheaper to buy new rods that come bushed & with high quality bolts.