What is the best blank and components for 9', 4 piece, 11 wt., Salt Flyrod for Tarpon? Never built a salt flyrod and appreciate assistance in planning for client.
What guides/ components are best suited for the saltwater conditions? How large (long, diameter) should cork grip and reel seats be? Brands that stand up best
in salt conditions? Any precautions in crafting such a rod?

Thanks ... Sam L. Slack

Did your customer specifically request an 11 weight? 10 and 12 weights are more common.

Grip = Full Wells (Careful about adding a fore or fighting grip unless the blank manufacturer builds its rods with one or advises that a fighting grip will not void warranty. One of the majors (maybe Sage?) offered a salt water grip that was a full Wells plus about 5 1/2" to 6" of fighting grip as a single grip unit.)
Reel Seat = Anodized aluminum that will accommodate a fighting butt and large reel.
Winding Check(s) = Anodized aluminum
Guides = REC Recoil or SeaGuide (and don't forget the tip top ought to be a large loop). I've had good results using REC reel seats and their titanium (Recoil) stripping and snake guides. Those who build using single foot guides might favor other guide manufacturers.

Some proficient fly-casters prefer extra-fast rods, some prefer moderate action rods, for some situations slow rods. If your client does not know what action he or she prefers I would go with moderate/medium action. A full wells grip configuration for sure but the ideal diameter of a custom grip depends upon the size of the user's hand. If your client didn't specify you probably should go with "medium". Some custom fly rod buyers are chiefly interested in wrap colors/patterns and component brand names, aka "appearance". Other custom fly rod customers are chiefly interested in performance/comfort. You have to sort them out.

Phil makes a good point on aesthetics vs function. Last rod I built for salt used REC Recoil strippers, which are, in my opinion, the ultimate for function, corrosion resistance, strength, etc., but have no visual appeal.

For saltwater you cannot go wrong with a graphite-nylon Fuji type reel seat. Same with the similar ones from American Tckl, Pacbay, Batson, etc. Bulletproof.