1973 6hp evinrude prop

[poohbear17721]

i need some help with selecting the correct prop for my outboard. I have a 1973 6hp evinrude. The prop on it now is a 7 1/4" plastic 2 blade prop. It will not get my flatbottom on plane. I saw there are a 7" and 8" 3 bladed aluminum. Which would be better to get on plane? From what i understand the 7 would give me more rpms but the 8 would push you further on each rotation. Any help would be appreciated.