After looking at the comments and original recipe, decided to use 1/2 Rhum Clement Canne Bleue (overproof white) and 1/2 overproof Jamaican Navy rum. The cocktail was not red, but had a good kick to it and wonderful flavours.
Bit disappointing. The soda water watered down the flavours too much so that all you could really taste was the slightly bitter funk of the Wray. Also, there wasn’t enough grenadine to turn it red - just vaguely pink. I reshook with 1/2 oz grenadine instead of a mix of grenadine and rich simple syrup, and reduced the Wray to 1 oz. Better, still not right though. I think it’s because the original recipe calls for a 151 dark rum, rather than Wray overproof.
Don Beach’s Shark’s Tooth certainly uses aged Jamaican rum – please see the link in “Variant” above. I’ve added Trader Vic’s original recipe which calls for the (I believe) defuncted “Ron Rico 151-proof rum.” Looking at online pics, bottles of this Purton Rican rum appear to be white, gold and dark in colour. How red your cocktail ends up will largely depend on how red your grenadine is.