I disagree with 75 mg promethazine being too much. It'll be sedating, but should synergize well (my favorite anti-histamine to take with opioids). Without a tolerance to antihistamines and naive as hell to opioids as a kid, I used to take four 'Meprozine Forte' capsules at once (the crap everyone used to have in their house from old dental surgeries) which contained 50mg meperidine and 25mg promethazine each. 100 mg promethazine wasn't enough to cause me any anti-cholinergic delirium, completely allayed any possible itching/nausea, and resulted in hours of nodding. And I unfortunately know well the signs of anti-cholinergic delirium, having experienced it on doses of diphenhydramine from 150mg up...
Not directed at anyone specifically, but note these old anti-histamines are often administered IV in hospitals and have horrible oral BA, so 75 oral isn't that bad. On the other hand, 25-50mg of IV dph or promethazine would knock most grown men out.