Many of them do have box mags. The problem will be finding one that's light weight in 7 mag. A 52 Jackson Hunter with flutes will get you close to the 7 lb mark, probably 7-1/4.Cooper rifles in stevensville montana. Rifle will run 2-3k depending on how pretty you want it, but they wont have a "box" mag.
"Browning Mountain TI". We have a winner. Good shooter and light. Should meet all your requirements. Good luck!
+1. A 300 WSM in an accurate ligher weight rifle is a very tuff combo to beat. I have 300 WSM it has replaced a 7RM for me .