While the stock market was busy bouncing, the heavy truck market headed upward in a steep and steady climb as final figures for March and the first quarter both scored solid double-digit gains.

The monthly U.S. retail sales figure of 12,454 heavy trucks reported by the Motor Vehicle Manufacturers Association made it the best March in three years, topping 1987 sales for the period by 13.4 percent and more than doubling the gain reported in preliminary figures.Sales for the first quarter are running 16.6 percent ahead of the 1987 pace, which was already one of the healthiest in recent years.

There's no question that demand for heavy-duty trucks is very good, said Larry Hollis, an industry analyst with Robert W. Baird & Co. in Milwaukee. Mr. Hollis is convinced this year will be a good one for the heavy truck industry. The only question is how good is it going to be.

Among the factors boosting truck sales, Mr. Hollis cited very nice growth in the manufacturing center, stimulated in part by the decline of the dollar, one of the same factors that has turned Wall Street skittish. But while the

dollar seems to be helping the truck market at current levels, Mr. Hollis warned that further declines could turn out to be too much of a good thing.

The market leaders in the heavy truck Class 8 segment, defined by gross vehicle weight ratings over 33,000 pounds, all recorded impressive sales gains for the month and the quarter.

Navistar International Transportation Corp. led the market with monthly sales of 2,843 Class 8s, an 8.6 percent increase over last March, giving the Chicago-based truck builder an 18.5 percent year-to-year gain for the quarter and 24.6 percent of the market so far in 1988.

Paccar Inc. of Bellevue, Wash., raised its total market share this year to 21.6 percent on strong combined sales at both its divisions. Peterbilt Motors Co. of Newark, Calif., sold 1,426 heavy trucks in March, driving a 67.8 percent monthly gain over the 1987 period and a 72.6 percent year-to-year increase for the quarter.

Paccar's Kenworth Truck Co. of Kirkland, Wash., retailed 1,370 Class 8 trucks last month in a 28.8 percent rise over last March for a 14.5 percent annual increase so far this year.

Although both now hold identical 10.8 percent market shares for 1988, Peterbilt's quarterly sales of 3,492 units edged out Kenworth's mark of 3,473 for the first time in recent memory in the intense rivalry between the sister companies.

Freightliner Corp. of Portland, Ore., with its 18.6 percent market share this year, also showed strong sales gains. The company sold 2,192 heavy trucks in March for a 16 percent annual increase and a 28.7 percent year-to-year gain for the quarter.

Mack Trucks Inc. captured 12.1 percent of this year's market with 1,683 Class 8s sold last month, a 2.2 percent rise over year-earlier figures, giving the Allentown, Pa.-based builder a 6.8 percent yearly gain on the quarter.

The new Volvo GM Heavy Truck Corp. of Greensboro, N.C., started to make its move in the standings, winning a 9.6 percent market share for 1988 with monthly sales of 1,342 heavy trucks and 3,106 for the quarter. Year-to-year

gains over sales of predecessor Volvo White Truck Corp. were 62.5 percent and 72.6 percent for the month and quarter respectively.

Ford Motor Co. monthly sales of 1,154 Class 8s fell 38.9 percent from last March, dropping quarterly results 36.1 percent from the year-earlier period. Ford holds 9.2 percent of the heavy truck market this year.

Combined General Motors Corp. sales of just 224 heavy trucks for the month and 701 for the quarter gave it just 2.2 percent of the 1988 market. Almost all GM Class 8s are now sold by Volvo GM.

Western Star Trucks Inc. based in Mississauga, Ontario, took 1.4 percent of this year's market with sales of 133 trucks in March, giving it a 19.8 percent year-to-year gain for the month and a 57.9 percent increase for the quarter.