Where can I find flight plan records for a private flight in the 1970s?
Depends on the type of aircraft, but if any records have managed to not be destroyed after 48 years, I would suggest you start by talking to the owner/operator of the departure and arrival airfields. US Customs might have a record of a customs inspection if there was one, but you'd have to file a FOIA request and be prepared to wait for an answer which may not be the one you want to hear.