Can't hold speed on even small hills

[Nobighurry]

Looking for some insight from those that know the 366. I have a 1970 C60 with the 366 (30k miles) and 4+2 465 trans. The truck is rated for 24k GVW and weighs 10.6k empty. I am using it to haul dirt and gravel but can't seem to keep any kind of speed on even the smallest of hills - empty or weighted. I have replaced / updated most everything without going internal - fuel tank and lines, filter, carb (edelbrock 1405), converted to HEI, new plugs and exhaust. I don't have any knocking or any real telltale signs of the issue. I've tried climbing in 4 low, but it is screaming and doesn't like it. It feels like it doesn't have the fuel to needed, but I am no expert. One of the first things I did was replace the worn out Holley for the Edelbrock and everything I read said to run the 600cfm. I'm wondering if it is not large enough to handle it???
It slows drastically with only 6k pounds in the bed and it should be able to handle twice that. Any ideas on what I am missing?