I suspect the rear axle ratio. (did the 64 have overdrive?---those were usually 4:30, straight 4 speeds were usually 4.10. Wagons were 4.56) Neither car had a tach, and shorter gears would make the engine seem livelier, as well as actually accelerating the car more rapidly. Both cars should have had identical B18D engines, 8.5:1 compression, 90 HP @ 5000 RPM. (in 67 the double downpipe was introduced, compression was increased a tiny bit, and output went up to 95 HP @ 5400) While a P1800 engine (B18B, 10.0:1 compresssion, 108 HP @ 5800, in 1964) would bolt right in, it's unlikely to have ahd one, as you know the entire history of the car from new. You would also know if the head had been heavily milled at any time.