What I'd do on this one is probably put the seam somewhere between the front of the door and the back of the wheel opening, because you can do quite a bit of sanding and puttying around there without hitting anything. Alternately, you could do it in front of the front wheel opening, so you're just grafting on the very front of the car.
I'd start by lightly marking a vertical line on both cars, at the same point, as accurately as I could. I'd then cut the front piece a little over a sixteenth of an inch back from that line, and the back piece a little over a sixteenth of an inch forward of that line, so that when you put them together, the overall thing is an eighth of an inch too long.
Then, clean up the edges with a file or sanding stick (or by sanding against a sheet of sandpaper on a flat surface) so they're vertical and smooth, and do a first test fit. Although it's still too long, you should have things fitting cleanly together, and you can check that it's sitting straight (rather than pointing up or down) and the same length on both sides. If it's not about right, then adjust it until it is. You don't need to be perfect at this point, just make sure things are in the right ballpark and make sure it will all work.
Next, carefully re-cut one of the pieces (probably the front clip) right at the line -- or, really, cut it just a little shy of that, and then sand it down to hit the line with a vertical and smooth cut. Test-fit things again, and again adjust the back half so that the front clip goes on straight and level like it should. Everything should be perfect except for being too long.
Once that's perfect, measure out exactly how much too long it is, and mark off that much to cut off the back half. Trim or sand down most of the way to that new mark, test fit again, and then sand a little more and test fit and repeat until it's straight and in the right place.
Finally, glue it in place, and glue a thin strip of plastic over the back of the seam to strengthen it. Add a little putty if the body lines don't quite match up, and sand the contours so it's all smoothly flowing from one part into the other.
Oh, and one more thing -- I think it's a good bit easier to do this with two styrene bodies, than if one is resin. If you can use a '72 coupe body rather than the resin El Camino, that might be easier (though if I'm remembering correctly, the coupe has a bulge over the wheel opening that the El Camino doesn't, so you might need to adjust the grafting accordingly).