its not so much an issue of how much HP the block can handle at all. its an issue of how radical can you make it & stay streetable(assuming thats what you plan to do) same with the tranny.
300 streetable HP is very realistic, & even fairly economical. 400 streetable HP may even be obtainable with a real good set of heads, a bulletproof bottom end & a roller cam, beyond that your streetability will suffer. 289s are great little motors, but cheapest HP is cubic inches. Oh by the way, I assume your talking normally aspirated horse power. John