wheel/tyre option for 500 SEC?

1985 500 SEC I just bought has 245/50.16 on rear & 225/50.16 on front One alloy is dented badly & spare is mismatch in offset Tyres are s*it What other sizes can I put on - standard is 205/70.14 This seems dodgy to me - how can a 16 replace a 14 inch? Anyone in Australia have a decent set to replace these?

thanks for any advice

Macgregor