Normally I would say you can create a checkbox on your lead/contact record in SFDC called "sync to Marketo", only check this box for your 40K leads, and then adjust the sharing rules so that your integration user between Marketo/SFDC cannot see leads where the checkbox is unchecked.
However, I did a quick check and it doesn't look like professional edition has sharing rules on leads! https://help.salesforce.com/HTViewHelpDoc?id=security_about_sharing_rules.htm&language=en_US
Double check this to be sure, if that's the case Marketo can create a sync filter for you on their side that functions in a similar way, but you will need to contact support.
As always test with small batches before doing your big import.
Justin Norris |
Perkuto