c5 engines can run 400hp VERY easily and the basic design was in EVERY way superior to the LT1.... including the aluminum engine block.
Cheapest? probably after you rebuild your engine, your intake and fuel injectors, ECM etc will not be up to the task of making 150 more horsepower so those will also needed to be upgraded... The TPI system was terrible at higher RPMs (I had an 89 GTA with the same engine almost) so that will pretty much need to be replaced, figure that the cost will be insane based only on the fact that hardly anyone makes go fast goodies for these any more.
You should keep your motor just in case you ever want it to be "numbers matching"