A quick and easy way to identify a small-block Olds is to look at the thermostat housing. The nipple that the upper hose connects to will come straight out the fron at a 45* angle and have a steel bypass tube that looks sort of like an inverted "J" with a small chunk of 3/4" heater hose connecting it to the top of the water pump.
As far as availability goes, I have never heard of an Olds gas motor being factory equipment (might be possible), but the 78-80 trucks came with 350 Olds diesels. It would be a simple swap to an olds gas motor as the diesel was externally identical to a gas Olds. If the trucks have Hydroboost, I'd put money on it being a conversion. The Olds diesel got a bad rap, I feel, quite undeservingly. They were actually a pretty good motor, but they suffered from lack of knowledge in regards to repair and maintenance. Very few people in that era knew much of anything about diesels, especially that they can't be maintained like a gas engine. This hurt their image quite a bit. Also, their injection pumps were not fantastic, but when maintained (there is that word again) they would actually live a fairly long happy life. I know a couple guys that had them and used them for commuting to/from work. One guy still has his. Other than (what we now consider normal--mainly changing fuel filters on a regular basis) maintenance and a few gaskets it is original with somewhere around 250K on it and it still runs great. He drove it across the country a few years ago and got around 23mpg with it.
Better or worse than a SBC of the same vintage?? In my opinion, an Olds motor is WAAAAY better than a SBC of any vintage. I have had the pleasure of owning several Olds powered vehicles and IMHO there is no comparison. The only thing I ever did to Olds motors (whether they were mine or a customer's) was at about 200K put a timing chain kit in them and maybe a set of rocker bridges IF they were worn or I (or the customer) was feeling rich--not that they were that expensive, but usually they were not necessary. That is it. I know several people who have over 200-300K on stock 350/403 Olds and other than some gaskets and a timing kit they are bone stock and still haul ass. Oldsmobile motors of the same displacement as a SBC always seemed to me to have a much broader torque curve that started at idle, but would still wind out very well. The only drawback to Olds motors is they are not a SBC, so parts for them are a fair bit more expensive and aren't so common that they can be bought at the corner convenience store.
