Looks to me like you are missing a bit of frame length. Maybe an inch.
Yeah, you should check out some of other 66/67 frames and see what they look like.
Personally, I'd hammer that beeyatch on there and weld it ... long as the bumper bolts up correctly.
Fit is kind of relative... to be honest the part is probably more correct in size/shape than your frame rails.
Most rear frame rail corners like that have from zero to 1/4" gaps.
In your case I'd check two things.
1. You are being too gentle with that part, bang that thing on there, it's a frame piece, not a fender
2. The frame rails are tweaked, bent, missing length.
Attached is a picture of my freshly sand blasted 68 frame a couple of years ago. Note the corner sucks, probably 1/4" gap, shiiiity welds etc.
So ... don't expect precision on the corners.