I haven't compared the difference between the ad_proc used in openacs 3.2.x and later versions of acs classic, but I think the -public and -private switches can be removed when porting. It sounds like they are just used to declare whether the function should be used internally in the tcl library or whether it is part of the public api which can be used by other modules. They might have some code in ad_proc that adds error checking code so that if a -private function is called from another module it will throw an error.
As far as the db_exec_plsql call, you are correct. You need to port "cal_ins_item_on_id_table" and then you can call it like this:
set item_id [database_to_tcl_string "select cal_ins_item_on_id_table(....)"]
You will also need to port the bind variables so that you use regular tcl variables instead.
