can someone give me a lead on how to research what vin's go to different engines? I'm going to go look for a chevy 350 in a junkyard but I can't seem to find the right info on what numbers are what.... 350, 307, 327 stuff like that. How to tell one apart from the other....

While that info is available, it's really not reliable. After 30 plus years, it's likely and probable that the original engine was replaced at some point. And finding a 327 isn't likely. To search for a 400 SBC, look for either 3 freeze plugs instead of the usual 2, or the casting number 509. To search for a 350, there were many casting numbers, but the most common is 3970010. This number includes both 2 bolt main and 4 bolt main. Next, heads. Pop the valve covers and be sure you don't find any casting number ending with 624. All the late 70s SBCs were painted light blue, so if you find one withlight blue peeking through any other color, it's probably been rebuilt then abused. All the late 60s SBCs were painted orange, and don't have the hardened valve seats needed for unleaded gas. A quicker way to rule out engines smaller than a 350 is by the size of the damper, though some 350s did come with the 6.75" one. All 400s and some 350s came with the 8" damper. You can tell if a block has been decked by checking the pad behind the water pump, on the passenger side. one ton trucks and corvettes were the original homes of 4 bolt main blocks, but these usually were abused. Best to get an angine out of a luxury car. If I've raised any questions, feel free to post them.

well seeing as it is coming from a junk yard, why does it matter how hard it's been beaten? as you said with most 4bolts, or the rebuilt then hammered again? Also what's so tabu about the casting numbers ending with 624?

624 heads are the most crack-prone SBC heads ever!
4-bolt-main blocks tend to have cracks in the main webbing, rendering the block junk. This is equally true of both 350s and 400s.

So would it be safer to pull a 2bolt motor and make it into a 4bolt myself? Also are the hardened valve seats really necessary in the 60's motors, I mean the only thing different about our gas now is the stupid ethanol...

The lead that was in the gas until they completely outlawed leaded fuel is what kept the valve seats from leaking, running unleaded gas in one without hardened valve seats causes them to wear faster.

Unless you're planning on turning the motor 7000+ rpms, a 4 bolt block isn't really needed, it's more of "hey, I have a 350 4 bolt main" type of thing.

Klortho is completely correct on both counts.

are valve seats something that tends to be costly? Also lets say I did get one, ran it with the old seats and they wore out; what kinds of issues would I be looking at? a rebuild, car fire or what?
I appreciate all the info you guys are giving me it's really interesting.

Head rebuild, with new hardened exhaust seat inserts installed, and hardened exhaust valves.