House-Senate conferees staked out tough bargaining positions on defense spending yesterday as they opened negotiations on a budget compromise to meet the Gramm-Rudman-Hollings deficit reduction targets for fiscal 1987.

Conference leaders from the Democratic-controlled House expressed optimism about reaching an agreement without excessive delay, but those from the Republican-led Senate expressed reservations and some apprehension.

"We have more similarities than differences," said House Budget Committee Chairman William H. Gray III (D-Pa.). "I wish I could be as optimistic as you are," responded Senate Budget Committee Chairman Pete V. Domenici (R-N.M.). Sen. Lawton Chiles (D-Fla.), ranking Democrat on the Senate panel, took a position in between, saying that the threat of automatic spending cutbacks under Gramm-Rudman-Hollings if deficit targets are not met amounts to a "shotgun at this wedding."

To no one's surprise, defense emerged as the critical issue almost from the start as the conferees gathered for preliminary posturing and laid the groundwork for real bargaining, which will begin after Congress returns the week of June 2 from an almost two-week Memorial Day recess.

While both houses settled for defense budgets that would level off President Reagan's military buildup, they wound up with a $16 billion difference in spending authority for the military. And that gap is complicated by a potentially difficult dispute over cash outlays for next year and the resulting balance between readiness programs and long-term weapons procurement.

Reagan had proposed spending authority of $320 billion, a $33 billion increase over current appropriations levels. The Senate approved $301 billion, the House $285 billion. Critics led by House Armed Services Committee Chairman Les Aspin (D-Wis.) contend that all three versions would squeeze readiness programs while authorizing continued growth in weaponry.

Although Gray had played down differences over defense, saying the two chambers are "not all that far apart," Domenici said Senate conferees have "a rather firm commitment . . . almost an instruction" to stick by their figure.

Several House conferees said the House is similarly determined to fight for its numbers. The House, said Rep. Marty Russo (D-Ill.), is "very, very vehement." Rep. Howard E. Wolpe (D-Mich.) also cited a "very strong" position in the House but also said he thinks that there is "a rather surprising degree of consensus" in both chambers about slowing the rate of defense spending growth.

Use of an anticipated one-year revenue windfall from the tax-overhaul bill to meet Gramm-Rudman-Hollings targets ran into strong reservations from key conferees, despite support from Senate Majority Leader Robert J. Dole (R-Kan.).

Gray said windfall revenues should be devoted to deficit reduction beyond the $144 billion deficit target for next year. Domenici said the Senate will continue to insist on $10 billion in new revenues beyond any windfall to meet the target.