Thanks for the quick responses guys,
On the point below, I assume its a easy process - I recall doing something similar when travelling to MIA on route to BOG but this was a little while back.
you'll have to clear immigration, then pick up your bags from the carousel, then immediately redeposit them on the re-check belt (they'll already be tagged to GUA), then pass through customs (handing them your slip).
Nux - flights were all booked through Delta for this but received an email a month or so ago saying it was being operated by Virgin. The joys of American airlines.
guv1976 - I doubt it. I was debating on getting an Uber straight to the beach - having a nice walk and then finding a spot for a bite to eat. Renting a car may take up a bit too much time in that 8 or so hour window?