Just keep a close eye on her, although those are big babies, there may be more. They can actually go 4- 6 hours before passing another kitten! Watch her to see if you see her pushing and straining with no kitten resulting.If you see that- then call your vet IMMEDIATELY!
You might just give the office a call and let them know a new family is in town and set up an appointment for tomorrow sometime so the vet can see the family for himself.
Congrats and thank you for not interfering with her to the point that she is more stressed than she already is. The purring is a reaction to the pain of the labor, as well as her way of guiding the kittens to find her right away- they follow her vibrations and then scent her nipple marking one as theirs.
It did my heart good to read this post! Congrats again! Also I would put a blanket over the top of the box. If she is in the closet- drape the blanket up over the clothes pole over head, and make a warm snug type teepee for her and the kits-