small block Chevy or GMC questions

[oldsub86]

Hey all, I hope you can help me with some questions about interchangability of small block GM engines.
I have a 1972 307 engine that needs to be either rebuilt or replaced. It has a deep knock to it and I am afraid it will prove to be pretty bad when dismantled.

So, I am trying to decide if I should just find another engine and rebuild or at least tear it down and have a look and get it ready to drop in in place of the one I have.

It is important that I be able to re-use all of the front accessory drive setup so I don't want anything to change on the front of the engine. It is also important that I be able to re-use the flywheel set up on the back of the engine without changes.

It really does not matter what displacement I end up with, so if a 350 would swap in, that would be fine. A few quick searches indicate that a 350 would be much less expensive to re-build with some pretty good parts than another 307 of that vintage would be.

So, my big question is: How interchangable are the blocks? Can I change in just about any small block GM engine and if not, what do I look for when trying to find something that will exchange? For example, there is someone locally advertising a 4 bolt main Chevy block. How do I know if it could be made to work? Casting numbers??

Will the existing heads and intake work with just about any block and if not, again, what do I look for? I would think that the heads and intake can be salvaged and I may as well keep them if I don't have to replace them.

I recall seeing books that set out the part numbers for interchanging this stuff but I never bought one when I had the chance.

I am prepared to go buy some parts but I would like to avoid surprises and not buy parts that won't work.

Thanks

Randy

PS - I need to change the line below as I sold the 68 last spring