Most 307s can be bored to 4 inches and You can make it a 350 ci. using a 350 crank and std.bore 350 pistons. I know of at least 5 that have been done that way. The only problem with that is if down the road it ever needs to be rebuilt again, you can not bore it any further. If you want a good performance street engine and You want to used the original block--Go for it. If you want a race engine, I wouldn't do that.