Michael Roderick Southall, a Northwest Washington dry-wall in-staller, was convicted yesterday by a Montgomery County Circuit Court jury of first-degree murder in the slaying of a 45-year-old Southwest Washington man whose body was found Jan. 31 in Silver Spring. Southall testified that he couldn't have committed that crime because he was selling drugs on the street at the time.

Witnesses in the four-day trial said that the body of the victim, Frank Wooten of 507 G St. SW, was found in his 1980 Cadillac in the driveway of 9015 First Ave. the morning after his death. Wooten, a construction foreman, had been shot in the head seven times and his throat had been cut, according to testimony.

Expert government witnesses called by Assistant State's Attorney Thomas Tamm testified that splatters of blood found on Southall's clothing matched Wooten's blood. Particles of glass also found in Southall's garments also matched glass from Wooten's bullet-perforated car window, other government witnesses testified.

Southall, who was represented by attorneys Barney Welsh and Joseph Trotter, testified that he was "a close friend" of Wooten's and had been with him the evening he was killed. But the defendant said that at 10:15 p.m. Jan. 30, the approximate time Wooten was shot, he was at 14th Street and Wallach Place NW selling the drug Preludin, which is used by heroin addicts.

Mary Elizabeth Waynes, currently an inmate of the Women's Detention Center in Washington, testified that she was selling drugs with Southall at the hour that Wooten was killed and said that after she went home that night, she called Southall and they talked on the telephone for several hours.