A 6 wt or a 7 wt will be adequate for fishing larger poppers, weighted crayfish, etc... But, when you say you're using Saltwater flies, do you mean your using them for bass or for saltwater fish? If you're using them for saltwater fish (Redfish, Bonefish...), go with an 8 wt. 5 wt's aren't made to make long casts with big poppers easily...
**I recommend taking a large #4 bass popper or heavily weighted fly and testing out a few rods...Then you can narrow down your possibilities...**