There is only 10 HP difference between a 305 and a 4.3L. To go from v6 to v8, will mean a ECM swap as well. If you are going to all that trouble, then a 350 is the way to go. The 350 is a lot better engine. For what you want to do though, 4.3l is most likely all you need. There are several automotive machine shops here where I live, that will build an engine like you want, at a reasonable price. I know there are some like that in FLA. Do some research, and find a good shop in your area, and take it from there. Very few mechanics do machine work. Most will pull an engine, and get an automotive machine shop do the machine work for them, and pass along this cost.