the 454 will make more horse power with less money, the diesel will eventually catch up in horse power and torque.The 6.5 isnt the best place to invest your money.
engines also are rated by sfc,thats specific fuel consumption,that is: an engine burns (X) amount of fuel expressed in pounds of fuel per hour based per horse power produced
example,not real numbers:
my 454 burns .5 pounds of fuel/hr/hp
so my 454 uses 50 pounds of fuel(8 gallons) per hour while generating 100 horse power
A 6.2 wont make 100 horse power
A 6.5 might burn 4.7 pounds of fuel per hour making 100 horse power
now its your math time