8.1 Vortec 496 Gas Or 5.9 Cummins Diesel For A 1983 C20 4 Door Dually With A Turbo 400 3 Speed Auto And 454 Carburated Gear Ratio 4:10

[Saint Fifty One]

For a 1983 C20 Dually 4 door with a 4:10 Gear Ratio, 454 Carburated engine, and Turbo 400 3 Speed Automatic Transmission. I'm trying to figure out what is the better engine choice out of a Cummins 5.9 Diesel or a 496 8.1 Vortec? 

 

The current engine such as the 454 has a light knock to it which i want to go with an updated engine with fuel injection, or if not a 496 Vortec then maybe a 454 Vortec. Along with decent if not decent MPGs and for heavy hauling. 

 

I know Cummins is the choice however. Althought for less the 496 is the choice and i heard it is simple to swap

 

I forgot to mention the tire size is 245/90/r16s with 4:10s, i dont drive fast past the speed limit by the way. Plus Unleaded gas is lower than diesel here in New Orleans, Louisiana