1984 z28

1984 z28 305 h.o. carb. 700r4
When i put it in any of the forward gears, if its just at idle or barely above idle i have power to drive, however if i raise the rpms I lose power and stop moving. Reverse works fine. I plan on checking fluid and going from there. However I don’t believe the fluid to be the problem. It was clean fluid the last time i checked, not even black. Looking for some ideas as to to where else i could be checking. worn out clutches or what? I would assume if the torque converter were causing this or fluid pressure was causing this, I wouldn’t have reverse