It looks like the 69 and 70 XL/LTD bumpers do not interchange. I think the centre point, or beak is more pronounced on the 70. Here is a page from the 63 - 74 Interchange Manual showing the grilles and bumpers.
The center point or peak on a 69 and 70 is more pronounced depending on whether the car has hidden headlights or not, the center point on a h.h. car sticks out more to accept the vacuum canister behind the bumper that actuates the headlight doors. I "think" a 69 and 70 will interchange if you keep that detail in mind but I have never had a 70 so....
The bumpers are not the same, but that doesn't mean it couldn't be made to fit. If you do a Google image search for both you can see the center portion, at the beak, in the 70 has a drop, where the 69 has a constant height on the top line of the bumper.
From the pictures 70xl posted the difference at the center looks minor enough it would pbly look alright. Ford being they way they were who knows what differences there are diminsionaly. If it would help i can take some pics and/or measurements of my 69 h.h. bumper. I also have a 69 bumper from a car without h.h.
On the 70 the centre grille section is taller than the side grilles, with the extra dimension being at the bottom, hence the drop in the bumper. On the 69 the centre grille is the same height as the side the side grilles. The 69 bumper may cover the bottom bars of the centre grille section on a 70 car, certainly the gaps wouldn't be consistent.