I have a friend in Oberon NSW that builds racing engines. We have been friends for 40+ years and grew up together building cars. He is building a new 308 for me for one of my vans and his attention to detail is impeccable.
I'm not going to quote prices because I'm not sure what he charges but I know whatever he builds will be a strong, reliable motor. He would be able to rebuild your 253 or find you a 308 or 253 at a price you could live with.
If you'd like to PM me I can put you in contact with him.