Are you really 19 years old?
You are probably looking at the book figures for aircraft that are at MTOW. If you want to haul the same load with a C404 as with a C402C then the C404 will perform better because it won't be as close to its MTOW.
Grossed out, yes, as I mentioned, the C402C on paper, is the better performer. It only has 2x325 h.p. where the C404 has 2x375 h.p., but the C404 has a much higher MTOW. Just divide the C402C's MTOW by 650 and the C404's MTOW by 750 to see what I mean, when it's excess power that makes one go up, the usually desired direction after take-off.
I flew five or six different C404s, including the last one off the production line. That one was an absolute pig! Go figure.
Those STOL kits promise the Moon, but again, those are book figures for an airplane with new engines, tires and brakes, some hotshot at the controls under optimum conditions... that might not be what you get in the real world.
There, the way to go, if you can afford it, might the bigger C404 not loaded to the gunwales, when you have more of a margin for error.
Better yet, forget this piston-banger BS and get yourself something with turbine power. A Twin Otter with PT-6-34s, made all shiny-new for a mere $4 million? Or ask Viking how much a brand new one is. Make sure you are sat down when they tell you the number, though.