I read the 6.4 blocks have better cooling and bigger bore capabilitys than the 6.1 blocks, is this true? if so why not run the 6.4 block even though they are vvt?

2000 Dakota R/T, 408 magnum, 727, Indy heads
1000cfm 4150 carb, 93 octane fuel.
motor; 10.258 @ 132.78
200 shot; 9.262 @ 144.69
racemagnum