This is just my opinion. These trucks are not worth a whole lot of money, so to me it is more important to build the truck that I am happy with rather than worry about original parts.
On these trucks the more important questions to ask is what is the condition of the roof, pillars, hood and bed sides? Until recently they didn't even make patch panels for these parts and there are no steel replacement parts.
Lay on the ground and look at the bottom of the fenders near the mounting bolt. People like to glob mass amounts of body filler in there instead of replacing or fixing. Do the same for the rocker panels and rear cab corners. On the rear cab corners you can pull the seat forward and stick your hand down inside them to feel for any filler. The last place to check is the passenger inner fender. The battery sits on top of this a rusts it out.
I'm not saying that if you find these problems to run but look at what you are paying for and decied if it is worth it. Basically I wouldn't pay $10K for a truck with these issues but at $4-$6K it would be worth it.
Remember paint can hide alot so look at these areas before you plunk down a huge wad of cash. The interior of the truck looks pretty good. Looks like aftermarket or dealer installed A/C and a B&M truck Megashifter.
To find out what motor you have you need the cast number on it
http://www.strokerengine.com/SBCcasting.html . It is hard so see with the motor in the truck. It is located on the rear top drivers side of the block. You'll need mirror at least to see it.
Another option is to take a valve cover off and the the head casting number. As long as it hasn't been changed, it should let you know what block or blocks it was used on.