converting unique index into primary key

I need to convert an existing unique index on a very heavily inserted table
into a primary key.  Alter table works, but locks the table for too long.
As a hack, can I just set pg_index.indisprimary = 't' and
pg_constraint.contype = 'p' for the appropriate row in each?  This is for
7.4.6 and 8.0beta3...

TIA...

Ed