225 slant rebuild for more power

If you want new valve seats, you're "gonna have to go through with that" to the machine shop. That said, the slant can be pepped up, but it will never equal a V8 for power.........without forced induction or nitrous. If you can live with that, then have at it. Staying naturally aspirated, you can probably see one horse power per cubic inch with mods such as bigger carburetor and intake, hot camshaft and headers.