The manufacturing sector's likely winners and losers began cautiously to take sides yesterday on President Reagan's tax proposal, as they also began the complex calculations needed to pin down the plan's impact.

The primary losers, Treasury officials indicated, will be basic manufacturing industries such as steel, autos and other equipment producers, that were able to take full advantage of the rapid depreciation provisions of the 1981 tax bill. If the accelerated depreciation provisions are scaled back and the current investment tax credit is eliminated, as the president has proposed, these companies will be the hardest hit.

Reflecting this concern, a Ford Motor Co. spokesman said yesterday that the company "supports the goals of making the tax code simpler and fairer and lowering marginal rates. Nevertheless, as we understand the broad outlines of the proposal, it appears that our nation's already declining industrial base may be targeted for a major tax increase.

"Certainly, elimination of the investment tax credit and the scaling back of depreciation deductions will have an adverse effect on capital formation and the creation of new jobs in the industrial sector," the Ford statement said.

But the president's plan is receiving strong support from other companies that have not had to invest as heavily as basic industries have, and that have not enjoyed the full effect of accelerated depreciation and the investment tax credit.

Next week, the chief executives of a half-dozen companies who support the president's plans will testify before the House Ways and Means Committee, including John M. Richman, chairman of Dart & Kraft Inc.

"Our current system has the effect of favoring one corporate taxpayer over another, one industry over another and one form of investment over another," Richman said earlier this month.

Richman has helped assemble the chief executives of a diverse group of companies who share his goal of a simpler, more even-handed corporate tax system -- among them General Motors Corp., J. C. Penney, Procter and Gamble, General Foods and Minnesota Manufacturing and Mining Co., Merck and Co., and R. J. Reynolds.

Tom McHugh, vice president for taxes at Dart & Kraft, said yesterday, "I think there remains among the members of the group I've talked with a strong commitment to fundamental tax reform. Everything I've heard is most complimentary of the president's initiative to undertake this as a personal challenge . . . to bring about some fairness and equity.

"I don't think I've heard of any corporation that's been untouched by the proposal. We will lose a great deal by the change in the depreciation. We aren't perhaps as capital intensive as some other companies and so it's not as dramatic an impact," he said.

While corporations that invest heavily in plant and equipment would lose major tax breaks under the tax-simplification plan, small non-capital-intensive businesses would see their tax bills shrink as the top corporate rate is cut from 46 percent to 33 percent.