X-heads vs J-heads: what's the difference??

Casting #3418915 'J-Head' ~ {1971/1972 Castings}

___________________________1971 '340'________ 1972 '340' ~ 1972 '360'

Combustion Chamber CC's.......... 69.0 to 73.0 .................... 69.0 to 73.0
Recommended CC's........................ 63.3 .............................. 64.7

Intake Valve.................................. 2.02" ............................. 1.88"
Exhaust Valve................................ 1.60" ............................. 1.60"

Intake Flow @ .400" Lift............... 193 CFM ........................ 190 CFM
Exhaust Flow @ .400" Lift ............ 137 CFM......................... 126 CFM
Intake to Exhaust Flow................. 71.0% ............................ 66.5%

Intake Flow @ .500" Lift ............. 200 CFM ........................ 196 CFM
Exhaust Flow @ .500" Lift ........... 140 CFM ........................ 129 CFM
Intake to Exhaust Flow ................ 70.0% ........................... 66.0%