DGT,
My 69 P11 is 3 3/4" X 6" outside to outside dimension.
Brazed on the Chrome-Moly frame. If yours is a 67 chances are you had magneto's which didn't need a battery to start. It may still have had the battery tray or tool tray??? as I'm not 100% sure.
I don't think they changed much in the frame during the P-11 series, again, I could be wrong.
Cheers,
Tom