It's probably the carburetor, but I'm going to complicate things a bit.
I suffered the same issue with my '49 216 with Rochester carb. I often run the interstates and keep up with traffic just fine. On hills I just hold the pedal to the floor to squeeze all the power I can get. On one such trip I found I was losing power going up a gentle hill with the pedal to the floor. Not normal. I found that if I let out on the accelerator a bit I would actually gain power. She ran like a little champ, but once the accelerator went past a certain point I'd noticeably lose power.
I gave the old truck a good tune-up and now she runs like a champ again with full power all the way to the floor. I never touched the carb, though.
