They were the standard motor for GMC's in 60-66 and went to 1974. They are boat anchors BUT VERY reliable. You would only want one if you are a GMC purest or want to maintain a stock truck. Max RPM is about 3500. About half of a SBC. They are very cheap to buy. No many want them. DO NOT buy one that needs rebuilt. Parts are getting VERY hard to find. I bought my 305 V6 running good with transmission for $200. There is still a good supply of those motors if you are patient. The hard part is finding one near you. They were built like no other motor, to last a long time. So be picky and don't pay much.....or switch to a SBC or BBC.
There are specific letters after the 305, like A, B, C, D, E, depending on year of truck. They are all basically the same motor with slight carb and other changes (125 HP to 142 HP). There are larger Magnum V6's also. The number is stamped on a pad in front of, and below, the passenger side valve cover.https://pueblo.craigslist.org/pts/d/pueblo-1961-gmc-motor-and-trans/7019566540.html