Clay Britt of Rockville (Md.) Municipal Swin Club qualified for the Pan American team tonight by finishing second in the 100-yard backstroke at the National AAU senior swimming championships.

Britt, 18 led, at the 25-yard mark but was overhauled, at the 50-yard turn by winner Robert Jackson of West Valley Aquatics, Los Gatos, Calif. Jackson won in 49.58, Britt was timed in 50.12.

Britt will be a member of the U.S. team which will compete in San Juan, Puerto Rico, the first two weeks of July.

Tami Paumier of Columbia (Md.) aquatics, who qualified for the Pan Am team earlier in the meet in the 100-yard breaststroke, tonight finished fourth in the 200-yard breaststroke in 2:17.65.

The Starlit aquatic Club of Fair-fax, Va., 800-yard freestyle relay of Brendan McCarthy (1:38.64). Phil perdue (1:38.95), Sam hewson (1:40.42) and Shannon Varner (1:41.02) finished sixth in (6:39.03). Florida Aquatics won the event in American record time of 6:25.42.

Only one American mark fill tonight as the record-shattering pace of the first two days, when nine standards were set, slacked off.

Steve Lundquist, 18 , of Jonesboro, Ga., won his second event in American record time, swimming the 200-yard breaststroke in 1:59.18. Lundquist had won the 100-yard breaststroke in 54.77.

Lundquist has been training for this meet only three weeks, having spent the nine months since the World Championships in East Berlin playing football and lifting weithts.

The 6-foot-2, 165-pound high school senior said after tonight's record, "My dad hated me playing football and all my friends told me I shouldn't do it but I needed a break from swimming."

Lundquist wasn't the only swimmer to win from the outside lanes, considered slower because of greater wave action. Glynn Perry of Florida Aquatics won his first national championship taking the 200-yard butter-fly form lane eitht in 1:46.89.

Two newcomers broke into the women's championship circle. Mary T. Meagher, 14, of Louisville, won the 200-yard butterfly in 1:56.58 and Patty Spees, 18, won her first national title taking the 200-yard breatstroke in 2:15.81.

Linda Jezek of Mission viejo, Calif., won the 100-yard backstroke, well off her American record of 54.94, in 55.67.