The one part of your question says "crank". If you are getting that deep into the engine why not simply drop in a 350? You will spend more money trying to get the 307 to the same power output as a stock 350 that it doesn't make financial sense to me. Now if you were simply talking about external bolts ons, that is a different matter. Headers, gears, a mild cam and good ignition would be where I would start.

Don