Just an educated guess, but it seems that the tube grille bolts to the outer two core support uprights. I can't really tell without measurements, but it sure looks that way from the pics. This is not the stock setup, as the original piece uses 6 screws and plastic screw clips - 2 in each support, top and bottom. If you've been driving around without the grille for any length of time, the clips themselves have probably fallen out. Not that it matters in this case...although you will need new clips if you ever find a stock grille and want to use it.
Anyway, you're probably going to have to remove the radiator panel so you can access the core supports from the top. Then it's just a matter of mocking up the grille in place, marking the location of the new holes, drilling the support and bolting the tube grille in place. You might be able to use some clamps to hold the grille in place while you're working.
If your truck has an interior release for the hood (I'm not sure if this was an option in '79), then you're all set. If it has an exterior handle, you're going to have to rig something up so you can get to it, since the top tube of the new grille may now be in the way. If the grille leaves you with a little room to move it up and down, you may want to set it low, which might leave enough space to get your hands on the hood release. Or you can grind/cut an access slot in the upper tube. Just be sure you have a way to get to the hood release before you close the hood.