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

Preferred Heads in Order

1} ~ #2531894 'X-Head'.......... mid-1967 thru late-1970 Casting

Note; Called the 'X-Head'
Note; Generally considered the 'best' 340 Cylinder Head. Consistent 'Port Measurements' of 160 CC's Intake and 70 CC's Exhaust.
Note; The 'tightest' Combustion Chambers with levels between {67.0 to 69.0 CC's}.
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2} ~ #2531894 'X-Head'.......... late-1970 and 1971 Casting

Note; Also with consistent 'Port Measurements' of 160 CC's Intake and 70 CC's Exhaust.
Note; These later 'X-Head' Castings will have approximately 2.0 CC greater CC levels, usually between {69.0 to 72.0 CC's}.
Note; When these later Castings were machined by the Factory, less material was removed from the Head Deck Surface than early Castings.
Note; This was done to lower the 340 Engine's Compression Ratio from 10.5-1 to 10.3-1 to meet the EPA's Emissions at lower RPM's.

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3} ~ #3418915 ....................... 1970 '340 T/A' Casting

Note; Typically called the '1970 T/A Heads'.
Note; Specifically cast for the 1970 Dodge Challenger T/A 340 'Six-Pack' and 1970 Plymouth AAR Cuda 340 'Six-Barrel' engines.
Note; The Intake Push-Rod Guide holes were bored further away from the 'outer' Side-Wall of the Intake Ports.
Note; This was done to allow for Racer Modifications, to allow for more material to be removed from of the 'outer' Side-Wall sections, which will increase Port Volume and CFM Flow.
Note; Special 'Off-Set' Rocker Arms were required for these 'T/A Castings'.
Note: The Combustion Chambers of these 'T/A Castings' usually came in at {69.0 to 71.0 CC's}.
Note; The Intake Ports were a miniscule amount tighter than the 'X-Heads' at 158 CC's.
Note; But, the Intake Port floor of the 'T/A Casting' had a slight improvement between the Bowl and short-turn Radius.
Note; The Exhaust Ports on the 'T/A Head' were a tad larger in Volume at {72 CC's} than the 'X-Head' at {70 CC's}.
Note; The Exhaust Ports on the '1970 T/A Head' do have a very slightly better Flow-Rate than the 'X-Heads'.

Note; Despite the slightly minor differences between the 1970 'T/A Head' and the 1968-thru-1971 'X-Head', both Cylinder Heads delivered similar Flow-Rates.

Misconception; It was thought at one time, that the '1970 T/A' Heads were 'specially designed' and were Cast with larger Intake Ports. Which is incorrect.

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#3418915 'J-Head'.......... 1971 Casting {w/2.02" Intake Valve}
#3418915 'J-Head'.......... 1972 {340 Engine} or 1971/1972 {360 Engine}