Default Cast type

Hello,
how can i choose a default data type for casts?
PG 7.4 has some problems to use an existing index, if the datatype not
match.
For example i have a int8 primary id column and search for this column,
i can't use the index for queries like:
SELECT ... WHERE id = 12345
if i'm use:
SELECT ... WHERE id = 12345::int8
it will use the index.

Is there a solution for PG 7.4 to choose the default datatype for this
way of casting?

--
Markus Schulz