coulda been a 273 or perhaps, 318 poly as they were both small blocks and both available in 1964 depending on the car (Valiant, Belvedere, Barracuda, etc). Neither was a big block.
318 Poly had heads with valve arrangements "similar" to the hemi's but a lot less complex! The heads themselves were big, and the valve covers were wide, and people sometimes incorrectly referred to them as 318 big blocks.
340 was a small block
360 was a small block
350, 361, 383, 400 were a big block and oddly enough the 400 uses a larger bore than a 440,...then there was also a 413, 426w, 426h--they were all big block but most of them came later than 1964
ain't never had a Mopar, don't really want one but if one fell into my lap, I wouldn't kick it to the curb. I've always stuck with oddball Ford stuff cause that's what I know. Being that I have spent most of my life around the drag strips, I've seen and learned a little about the Mopar's.
My friend (now passed away) ran a '56 Ford drag car since the day he bought it new in 1956. Started out with a 272" Y block, blew it up immediately, then a 312, then back to a 292, never could keep them going. In 1967 He found a 428 CJ and threw that in, won a LOT of races out in California with it, destroyed it, found a 427 and used it for a while til it let go (all bottom end failures). Moved out to Arkansas & settled down in the sticks, bought the car with him and no place to really race it so it sat for a number of years with no engine. Then he found a drag strip, and got the itch. Couldn't find anything but 390 truck engines but found a 440 and TF727, and put that in, and ran it for YEARS, up into the 2000's. Blew several of them up but always was able to fix them. One of them, kicked a rod out of the bottom of the block and obviously knocked a hole in the side of the skirt so rather than use up another block, he took an old cast iron skillet and cut it big enough to cover the hole, and brazed it over it to seal it up. It works for a good number of years until he got too old and sick to really do much work to keep it going, passed away a couple years later. His boy's still got the '56 Forge victoria (Ford with a dodge engine) and has plans on making it great again.
There are people who come into your life and people that go, some of them you just kinda forget about. He's not one of those.