You would need a block made according to the cast time stamp up to and around 3 months of your build date (obviously not after).
For the real engine number, it is written in the Owners Manual/Service Policy and is why car with them are more desirable and likely more valuable.
I report from www.holdenhistoricalservices.com.au in some cases will let you know the engine number (if it was recorded).
Being a HJ 308 the engine number will start with QT.