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

Casting
#2531894 'X-Heads' {1968 thru 1971}....................................#3418915 {1970 340 T/A}

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

Intake Bowl Size......................... 1.75" {87%}......................... 1.75" {87%}

Factory Combustion Chamber...... 67.0 CC's to 69.0 CC's........... 69.0 CC's to 73.0 CC's
Recommended Minimum.............. 63.5 CC's.............................. 63.5 CC's

Intake Port.................................. 1.16" x 2.27"........................ 1.16" x 2.27"
Exhaust Port................................ 1.25" x 1.75"........................ 1.25" x 1.75"

Intake Port CC............................. 160 CC's............................... 158 CC's
Exhaust Port CC........................... 70 CC's................................. 72 CC's

CFM Flow @ .500" Lift.................. 225 CFM............................... 228 CFM
CFM Flow @ .500" Lift.................. 147 CFM............................... 146 CFM

Intake to Exhaust %..................... 64.5%................................. 64.1%

The 'X-Head' is generally considered the best 'Stock' Cylinder Head, as the Combustion Chamber is a little tighter than the 'J-Head'.

Both 'Ports' are near identical in volume. But the Intake Port on the '1970 T/A Head' has a slightly better {miniscule} better floor design.

The 1970 {340 T/A} have the Intake Push-Rod holes bored 'off-set, which are designed to allow for added Racer Modifications to open up the Intake Ports.