When cracks developed in the floor of the U.S. Customs Service's parking garage in Miami, thanks to the tons of confiscated marijuana stored there, District Director Winston E. Pitman decided something had to be done.

The result of his complaint was the issuance Friday of a final rule modifying the decades-old requirement that Customs officials must give the public three weeks' notice before disposing of confiscated material.

The Customs Service rule was designed to prevent the government from selling confiscated property, such as jewelry that someone was trying to smuggle into the country, before the owner could claim it.

But since few people legally are allowed to possess illegal drugs, a person making such a claim would be inviting prosecution for violation of drug laws.

And stepped-up federal efforts to curtail drug smuggling in Florida meant confiscated drugs, mostly marijuana, were coming into the Customs' Miami storage rooms faster than they legally could get rid of them.

In fiscal 1981, 3.4 million pounds of marijuana was seized in the service's Southeast region. Late that year, the service was forced to rent two 18-wheel trucks (at $160 a day) to store the drugs and assign Customs personnel to guard the trucks in the parking garage.

Last May, the service relieved the pressure by issuing an interim rule allowing immediate destruction of such confiscated drugs as marijuana, heroin, peyote and psilocybin.

Friday the rule was made final.