The Z20 or Z24 engine is a great engine but it sounds like the head gasket is blown, the cylinder head is cracked or there is serious internal engine damage. Your own engine would need to be removed dismantled cleaned inspected and rebuilt or a re manufactured engine installed all of which may exceed the value of the truck. The price of a re manufactured engine will vary greatly depending on weather the core from your engine is rebuildable and what warentee is offered with the engine. I can only suggest calling around locally but remember the cheapest rebuilt engines available are not the best quality varies a lot. Ask your local garage if they have installed that brand of remanufactured engine before, had they any problems and what warentee would you get with the job.