66 GMC What engine?

[67chev"BIG-Bertha"]

Hey everyone, so my wife just bought a 66 GMC. It is a pretty fun truck, and after passing newer vehicles on the freeway, I was loving it, unfortunately I blew a tire about an hour from our house, so I gimped it along to the closest gas station and asked a guy if he had a jack and lug wrench I could borrow since I just bought the truck and didn't have one. Well, I got the spare on and made another half hour and blew the spare. So it sat overnight on the side of the road until I could get it home the next day.
Anyways, we got it home and drove it around a little. We decided to change the oil so I drain the oil and start taking the filter off, and that sucker would not budge. I had the standard filter wrench on there, and it just crinkled up. So I tried pounding a screwdriver through the side to twist it off, and nothing worked, I tried for about a month on and off trying to get it off. Finally, I took the part off that the filter screws to, but I had to basically cut the filter to pieces to get the bolts out. I put the filter in a vice and heated up around the threads and finally got the filter off!!! Next I went to my local NAPA to get the right filter, and after measuring what was left of the old filter, we got a filter that fits. The computer says that the filter for my wifes truck is for older tractors. Needless to say, I am stumped as to what engine this truck has in it. I know it's a V6, and someone told me it is the Big Block V6. I might be stupid, but I had no clue there was such a thing as a Big Block V6. If anyone can help me figure out what engine I have I would appreciate it. I'll try to get a picture of the engine posted on here. Oh yeah, I forgot to mention I believe it is a 3/4 ton truck because it has 8 lug rims.
Thanks everyone.