the info will be on the block just in front of the passenger side cyl head. there are no eng suffix codes on the heads. as Bart says it may be a replacement eng and in a lot of instances it was cheaper to drop in along block rather than get the old one rebuilt,(downtime and the longevity of the rebuilt versus new). in mid duty trucks of those vintages if one got 125,000 mi. out of an eng, replace it. you would never seem to get another 125,000 out of a rebuild but new gave it to you.
your 64 had the 327 installed in it as a replacement eng, whether used or new
ron
