You need casting numbers on back of block, top side. AND serial number on machined stick-out pad, front, pass side at head/block join.
http://www.nastyz28.com/sbchevy/sblock.php or google mortec. Head cast numbers are under the valve covers.
The blurb on that site says matching means matches that vehicle. You can't "actually" be sure it was built with that exact motor, but you can see if engine components were built/cast in the same time frame as the truck. That's numbers matching. Another point they make, which I agree with, is numbers matching on a 66 pick up is only mildly important as compared to a super valuable muscle car, etc. But I would keep the 283. If it's numbers matching, all the better feeling. Post your numbers if you want help. Make sure you really check out a builder until you get a good feeling from him. It ain't cheap. You may find that the block or heads are beyond repair, in which case you will have to find "numbers matching" parts or just build with what you can find. So be advised that the goal for 100% numbers match could get scuttled. Put an aluminum modern intake on it, a four barrel and dual exhaust. Up's the performance greatly at small cost. Leave inside of motor stock. Do not run the new engine until you are sure gas tank and all lines are clean clean clean. This is all my opinion, not meant to be the absolute only way to do it.
