X frames were used through 63 I believe.....64 was a milestone year for GM.
Bubbletops are absolutely beautiful cars especially a 60 caddy 2 door HT. Want one so bad but too friggin poor to get one LOL.
Chevy used the "x" frame from '58-'64. Interesting side note, the Pontiacs of that body style used full perimeter frames. (Got a friend with a '61 Ventura, tri-power, 4 speed, went to rack it up and got all confused).