General Motors Should love stock car racer Buddy Hopkins of Potomac. He happily put a Chevrolet engine in the Oldsmobile he is preparing for Friday night's $10,000 King of Kings 100-miler at Beltsville Speedway.

"It's a matter of weight," explained Hopkins. "Late model sportsman class cars must weigh at least 3,200 pounds, or nine pounds per cubic inch engine displacement. By using a 350-cubic-inch Chevrolet, which is competitive, instead of the larger original olds, I must add on about 400 pounds to make the minimum weight. The important thing is that I can add the weight where I want it to improve the car's handling."

Hopkins pointed out, as all racers know, there is little difference in the basic GM engines. "The body shell is different, of course," he added. That is why the welcome variety of cars in the King of Kings is deceptive. Most will have Chevrolets under the hood because they put out more power for their weight than other engines and parts are easy to get.

The field includes Grand National stars Darrell Waltrip in a Chevrolet and Lennie Pond in an Olds. Harry Gant, Taylorville, N. C., second nationailly in this class last season has a Buick Apollo and Canadian Earl Ross a Pontiac Ventura. Local favorites Jack Bland, Bobby Ballentine and Reds Kagles drive varieties of Chevrolet.

Hopkins literally walked into his Olds ride, the first of the make to race locally in some 15 years. "I went to Farrish Olds in Fairfax to buy a jeep," he recalled. "Then I got to talking to Mr. Farrish about racing and he was interested. He offered this '74 Omega I'm racing.

"We haven't had much time to prepare this car, so we are working mostly on the suspension and the handling. We'll have to use our old engine because the new one isn't ready yet. We'll just hope it gets the job done."

The "we" is Hopkins' partner and mechanic. Dick Lowe of Brentwood; Lowe's two sons; Jerry McManus, the sheet metal man, and Jack Fletcher, who Hopkins calls "the boss who keeps us all going." They have been a team for a little more than two years.

"I had been racing on and off about eight or nine years when we got together," said Hopkins, who will admit only to being in his 30s. "Back in 1974. Dick asked if I would like to try his limited sportsman car. I said I would and I did, but we didn't do so well at first."

Things were better the next two seasons as Hopkins won the limited title at Old Dominion Speedway in Manassas both years and Lowe was voted top mechanic. Those days are past.

"I've got a late-model class license now," Hopkins said. "I can't race in limiteds again." Drivers generally move up in class once they have proven themselves in lower divisions.

Hopkins has no great ambitions at present. He is typical of the weekend racers who enjoy the sport and the modest glory it brings.

He got interested in racing because his father raced on West Virginia dirt tracks. His own career began in a 1956 Ford "which didn't get around so good so I got a Chevy." He had to lay off racing a time or two because of accidents, he admits.

He is still a limited man at heart, which is why he is not entirely in favor of the local tracks mixing limited and the faster late models in the same race tO increase the field.

"I know the limited driver may make more money running fifth or sixth," he said. "He could be first among the limited, but no one would know it."

But money is not everything to racers like Hopkins. Winning is what makes them go round.