Re: VIN\'s
Truck VIN can be found on plate on the top of driver's side dashboard or driver's door pillar. Some people have also found a vin on top of the frame rail by the power steering pump (on older trucks).
If you search the web for "vin decoder chevy truck" you'll probably find a site that tells you how to decipher the VIN so you can find out what engine it had, GVW, 2 or 4 wd, 1/2, 3/4, 1 ton and the year of the truck. The 70-87 chilton's manual for these trucks also has a VIN decoder.
Also look in the glove compartment for a list of options that originally came with the truck. You can probably find the definition of these codes on the web too.
You're right about things being swapped in and out tho - the vehicle VIN and options sticker will only tell you the original goodies that went with the *body* (unless you find a the vin on the frame). My truck's VIN tells me it's a 1987 2wd 1/2 ton with a V6! That's probably why the body's in great shape - but it actually has sb400, th400, np205, 14bFF.
Your best bet is being able to visually identify the transmissions, transfer cases and axles. Advanceadapters.com has a good description of how to id lots of parts like the xfer cases and trannies. Pirate off road also has a good tech section that tell you what axles, trannies and xfer cases you can expect to find in a given truck.
Engines are a bit harder - a smallblock 305, 350 and 400 look pretty much the same. Big blocks are pretty easy to spot by the valve covers. The best way to id a smallblock is to take a rag and wipe the crud off the front of the passenger side head and block where the head gasket is - on the block surface there (facing up) and sticking out forward of the head will be some numbers that tell you what the engine is. Search the web and you'll find a quite a few chevy smallblock code decoders. I was suprised when I found out that my truck had a 400sb.
Given all the info on the web, the only difficulty I had was differentiating between a GM 10 bolt and a dana 44 front axle. All the diff cover gasket pictures I found on the web showed a rear 10 bolt which (to me at least) looks basically round. The front 10 bolt has a slope on the top driver's side like the dana 44 (which also has 10 bolts on its cover). Scrape the crud off the driver's side 'webs' on the diff housing and look for some letters. My 10 bolt had GM 85 instead of "44" or whatever Dana puts there (I'd guess the 85 means 8.5" ring gear). No bigge between these axles anyway - they're pretty similar strength-wise from what I hear.
It's handy to have printouts of VIN and engine code decoders along when you go to look at a truck.
