I'm looking into building a new engine and I can't decide between a 302 or 306. Are there any major pros/cons to building a 306? What kind of power difference would there be between the two?
Between the 2 maybe 5-7hp.... The only difference between them is the bore size, a 306 is a 4.03" bore which helps unshroud the valve a little bit more, other then no difference.
If you want to buy new internals, go 331 or 347 stroker..
A 306 is just a 302 with an overbore of .030". It might add a tiny bit of power, but the stock blocks are so thin that that will be the last rebuild that it ever has. There isn't enough metal left to go to .060" overbore, so you throw it away and get an new block.
There is nothing wrong with a 306, but if you buy a brand new block, there is no reason to bore it to 4.030" if 4.000 will work and leave you a rebuild or two.
327's and 342's are the standard bore versions of 331's and 347's. 331's and 347's are strokers that add displacement by making the piston travel further instead of making the piston larger. Strokers have another set of issues, but to make a long story short, there are two valid reasons to do a 331 instead of a 347, but both can be ignored unless you want to run high rpm's and if you do want high rpm, the 347 might wear out a bit sooner, or break a bit easier.
It is always a good idea to stick to a standard bore if you can, but for a high performance rebuild of a used block, most of the time you have to bore it.