Depends what you mean by safe...Those are fairly large doses, but assuming you have a decent tolerance to both drugs, you should be OK, in the sense that you'll live, but it's not really for anyone else to know how it might affect you. Some people really like the "speedball", although this typically refers to IV'd cocaine and heroin. In any case, it can be pleasurable, but it's also inherently risky, and I don't think anyone should promote using this way as it can often lead to dual addictions.
Have you combined amphetamines and opiates before? You'll want to take less than you'd usually take of each drug when combining them. Another major thing to note is that the opiate effect won't be as pronounced while you're still on the speed, but it will of course still be in your system - don't take more to achieve the same or usual effect or you'll seriously increase the risk of overdose. Also, there's no point in snorting diphenhydramine, just take it orally.