How much are you willing to spend on this motor? 250 hp is good for a cruiser, but in a heavy car like the 65 Chevelle, you will probably get bored with the performance after a while. You can get 350 hp for a great price with 350 crate engines from Edelbrock or GM Performance parts, plus they are assembled, tested and warrantied. This will save you grief from chasing parts as well as saving a lot of time building the motor. Also, a 350hp car will be a blast to drive and leave you with a big grin after mashing the throttle. Honestly, if you are buying a completely new engine, you will not have to spend much more for a 350hp 350 than you would on a 250hp one. However, if you want to start making 400 hp or more, the price really goes up because you start seeing roller cams, better heads, and other $$$$ parts. Getting 350hp is possible with conventional, production style components, and this usually translates into a lower cost. The bottom line, you will get what you pay for, so evaluate your budget before deciding.