A Vortec 454 makes ALOT more torque, especially in the lower rpm than a 6.0L. The 6.0 does not reach peak torque until it is close to or over 4,000 rpm depending on the year. The Vortec 454 reaches peak in the 2,800-3,200 rpm range. The older TBI 454 puts down its torque peak at 1,600 rpm and it matches what the 6.0L at that RPM.
I am not a diesel fan especially the newer electronically injected garbage ones. The newer they get the more of a maintenance nightmare they are.
The 8.1 with a stock tune is 455 ft/lbs @ 3,200 and picks up 50-60 ft/lbs of torque with a decent tune. Once you swap on some headers, port the intake manifold, put a mild cam in it, it easily cranks out 550-600 ft/lbs.