The 7.4L (454cu in) is an older designed engine than the 6L and wont get as good mileage as the 6L, but the 6L also doesnt quite have as much torque either.
The 454 (7.4L) has been around for a long time,(25+ years) and is a good engine, in my opinion, its biggest shortcoming is in cooling, you need to keep a close eye on the complete cooling system, and dont let it over heat.
Some years ago, they were real power houses, but with smog requirements, and fuel octane ratings, they have been detuned some. But there are a lot of them still out there. They might get 1 to 3 MPG less than the 6L, all else the same.
2001 27' Four Winds Class-C E-450 V-10.
Buick Park Ave Ultra, Ford Ranger PU, JD 500 backhoe.
1941 Farm All "A"
No substitute for cubes. Best gas mileage, depends on the application. Put em both in a car the 6 liter will win hands down, the 7.4 has a bigger stomach. Make em tow the Rockies to the Mighty Mississippi and the lungs on the 6 liter will be bursting.
I had a 96 truck identical to my present 97 except for the vortec 5.7 which had 4.10 gears. On a 30 mile one way trip to work, 2 lane hiway and city streets, the 5.7 got 12 mpg. The 7.4 with 3.73's got 13 mpg. All the hype on the Vortec series got me to try the 5.7 but the first tow I wanted my cubes back. I don't know what I'll do now that GM no longer has a big block.
2012 Chevy 3500HD Dually 4X4
Crew Cab long bed 6.0 gasser 4.10
06 Hitchiker II LS 29.5 FKTS
Peg Leg, switch to a 415 cu in Ford V-10, 30 valve, its the best of both worlds. Is 415 considered a big block or a small block? But the 365 hp and 460 lb ft of torque (stock) is not bad. And Im a Chevy man.
I wish GM had taken the 350 and added 2 cylinders and over head cam heads, I wonder how it would perform with 437.5 cu in.