GAINS TAX BREAK APPEARS AT HAND
Congressional conferees agree to add to Internal Revenue Service reform bill measure proposed by Rep Bill Archer cutting holding period to qualify for most favorable capital gains tax rate to 12 months from 18, retroactive to Jan 1; Pres Clinton's aides expect Clinton, who pushed for 18-month holding period in 1997 tax law, to sign bill; Congressional aides say provision will cost Government about $2 billion over 10 years; Republican leaders are now expected to seek cut in top capital gains tax rate (M)