Given the way that Gammoudi upped his game in the 1964 10000m (you think *Mills* was unknown before that race?), I have to think that Gammoudi could have come up with something extra in a 1968 sea-level 5000. God, what a pleasure it was just to watch Gammoudi move during his top years...
But bear in mind: Clarke in 1967/8 was absolutely aces and spades above every other distance runner--utterly dominant. I believe T&FN ranked him no. 1 at both his primary distances in both those years. (I won't look it up, so feel free to correct me.) He was capable at that time of running away from every runner in the world. In a sea-level 10000m he could have run the first half in around 13:45--though I would love to have seen such a race, because he certainly would have had some company (especially Gammoudi) and he wasn't always at his best in company.
Still, I don't think Clarke would have done that. In his book, he talked about being the fastest 5000m runner in a 10000 field, and how he had come to feel that the ideal tactic for such a man would be to run the first half of that race moderately, then go "bing." Running the first half moderately, against G and others, would simply have given them hope--no way (I think) that he'd have dropped them all in the second half.
Regarding the mitral valve (I believe it was eventually replaced with a pig's valve): Clarke collapsed at the end of the 1968 10000, something he had never done in his career, and for a time it was feared that he might die. (Recall Ryun's comments after the 1500, when he sat down for ~30mins--simply couldn't move--and honestly felt that his time had come.) Clarke really may have run himself into damage there, and his later participation in the 5000 seems unwise in retrospect.