Mid year 273 in 64 Darts?

According to Issue 59 of the Slant 6 News:

At their 1964 introduction the Slant 6 engine was still the only available powerplant for Valiants [and Darts, see Issue 33]. . . . The new 273 V-8 was announced in December, 1963 and went into production that same month. The first V-8s hit the dealers' showrooms after January 1, 1964. . . .

Don't know if that would be considered mid-year or not, but it wasn't available at the beginning of the 64 model year.