LSx will cost more to buy, but no money needed to build. T56 is a big heavy trans. SBC is cheaper than SBF, but don't think it will be under 420# (maybe under 550#) - it's about 100# heavier than an SBF. You can get to 350 pretty easy with SBF roller motor with aftermarket heads ($1,000 new) or grind away at the exhaust ports + decent cam and fuel delivery. A world class T5 will be about at its limit at 320-350 torques. They can be built for more though. Then again, it's enough power to just break the wheels loose - so less drivetrain stress.
other thought - toyota uz1fe lexus version at about 300hp and supra trans (forgetting the number - 156???).
dsm 4g63 turbo?
sbc conversion is well documented with aftermarket support. sbf probably as well.