THIEU DUE TO ASK NIXON ASSURANCE
S Vietnamese Pres Nguyen Van Thieu prepares to set out for US Mar 31; Saigon officials say his principal purpose is to obtain directly from Pres Nixon minimal assurances underwriting the survival of a non-Communist S Vietnam; believes he seeks US pledge to intervene with air power in case of a N Vietnamese invasion and a US guarantee of enough econ aid to meet the costs of reconstruction; opposition politicians say Thieu seeks pub raffirmation of US support not only for Saigon but for himself; actions by Thieu in recent days seen a conscious policy of refurbishing his pub image to make it easier for Nixon to confer the sought-for support; Saigon officials say Thieu intends to liberalize pol life after yr of wartime rigor and to make adm more efficient and to eradicate corruption; opposition believes moves by Thieu to be little more than tokens designed to attract US attention and hopes that Nixon will insist on more genuine liberalization; opposition leader Tran Van Tuyen comments