casting parameters to a function

I am trying to move all of the sql statements from my code into a pl/pgsql
function.  I have one function for a transaction block that contains 10
inserts and I need to pass 50 parameters to it.

Well I can't do that, so I was playing around with passing the parameters in
as a varchar[] array, but then I have the issue of the variables being the
wrong type when I insert them.

I was playing around with trying to convert types, but it isn't working so
far.  The function takes a single varchar[] as an argument..

This doesn't work:
s ALIAS FOR $1;
in_trans_date timestamp := s[3];

and this doesn't work either:
in_trans_date timestamp := $1[3];

Can someone help me in how to assign an array element to a variable and cast
it?