Just like we don't put $3,000 rims and tires on a $400 car or a $4,000 stereo in a $600 car we try and not put $3,000 heads on $150 motor...
If the idea is to build it over time and to build big street horsepower and leave what you have in there and start building it on the side from scratch. Get a block and get it to the machine shop and start buying parts for it... In the meantime start beefing up your rear end your driveline and your transmission to take what you're going to throw at it...
Or you can build it like a kid and make a massive motor that blows the tranny and then build the tranny that blows the driveline and then on to the rear end getting blown up and paying for a new one of those after the fact...