Since the engine is a transplant (from a 71 GTO you say), you need to be sure that it's what you think it is. All Pontiac engines from 326 CID to 455 CID are the same external dimensions and look pretty much the same. You need to check some stuff to see what it originally started out being. I note that the engine has tube headers, which are non-stock so there's every possibility that the engine's been tweaked by somebody.
The cylinder heads will have a number cast into the two center exhaust ports on each side. Those will be either 2 or 3 digits and/or numbers -- eg. "62", or "16", or "6X". Although the rust may make them hard to read, you need to know those numbers. If you can get those numbers, we can find out exactly which heads they are, and that will help determine what the engine is truly from. I'm not casting aspersions on the seller, but somehow engines frequently seem to come from cars that were "hotter" than the car the engine left the factory in. Unless you check the numbers, there's no way to tell.
The block will have the original displacement cast into it on the driver's side underneath, towards the front -- eg, "400", or "350", or "455". Also, the block will have a 2-letter designation stamped into the front of the passenger side, on the machined face right underneath the head. This designation will be something like "WT", or "YE", or something of the like. That will tell us what engine it was, and which transmission it came with originally.
On the passenger side rear of the block, you will find a 6 digit casting number that will read something like "500557" (it probably won't be that number exactly because that's a late 75-76 number, but you get the idea). That casting number, along with the 2 letter designation, will pretty much narrow down which engine it was originally.
In addition, there are some numbers on the rear of the engine under the distributor that you need to get. Those will give the date code of when the engine block was cast, which furhter narrows down what it is.
Now, keep in mind that ALL of this information only HELPS ID what the engine is NOW. It could have been bored and stroked to something larger (very common for them to be bored after all this time), and it's almost certain that it will have a different (non-stock) cam in it if it has been rebuilt. But if you can determine the information above, we can help you figure out what the basic pieces are and thus get an approximation of what you actually have in front of you.
