An auto is fine, my first Skyline was auto, I could eat any other 5sp with similer mods. You can launch at full boost for christ sakes, or install a stall converter.
As far as 300 or close to goes, you won't have to do anything aside from install a turbo and get some fuel and ignition tuning done. I could put you into 300whp for about 3000. And thats a reliable safe tune.
Email me if you have any questions, I'd be more then happy to help.
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WD