Marketo support gave me the official answer - I can't do it. Basically, if I'm attempting to display more than 60 "non-Marketo" fields (anything synced to an SFDC field; either on the opportunity object, or a custom SFDC field), it won't work. No workarounds. I brought up this thread and they said that if it's a standard Marketo field, it would work, but otherwise, there's no way. I'm not entirely sure I believe them, but they were pretty steadfast about it.
Earlier you said you were using 18 fields from the Opportunity object.
Where is the 60 coming from? 18 x (# of Opportunities)? Or 61+ fields on a single Oppty record?
Did you send email to a lead using the Email editor?
Did you write a smart campaign to send the test email?
Try the second way and see if that works...