Buy a rebuild kit for a 350 then replace the block with one that the parts will fit in.

The 305 won't hold up to a blower or turbo or nitrous. You need 4 bolt mains if you want to build horsepower and the 305 doesn't have them.

It'll run better with an Edelbrock performer cam and intake and headers but that won't even be close to the HP you want from it.

If there was a practical way to get 325 HP out of that engine it would have been done long ago and there would be a kit and lots of magazine coverage.

A lot of things have been tried to help these engines but you get much more for your money if you start with a better engine.
AL