I had debris in my anti siphon too. It is a 1.5" fitting at the fuel tank. I would not trust the shop saying they "fully" checked the fuel system.
I installed a permanent fuel gauge in-line just before the carb. This lets me see that I have fuel pressure and can help trouble shoot anything down line if needed.
The reason I suspect fuel delivery somewhere is power and rpm loss is only occurring under load. Unloaded, the demand is not as great and a partial blockage may be able to deliver enough fuel to not cause a problem.
1991 MariStar 240sc, 454, tower, WetSounds, Krypt and Rockford Fosgate