New oil pressure switch doesn’t fit...318 LA

You could use a step up brass NPT bushing (1/8-1/4). They usually have a flat so if it bottoms out then use an impregnated washer ( or a copper washer for that matter) to form a seal. Then use a step down bushing to 1/8 again to mount you're switch. It will raise it up a bit but you're light switch is shorter than the gauge switch so I don't think you'll have a clearance issue.