You should have left about an inch of the original guards infront of the seam. The inch of steel would be used to stop the new guards from flexing inwards.... you would just stick some thin rubber on it to stop metal-to-metal contact.
Leave the original seam and seam cover on the car, but take the seam off the new front. This will help make it look non-removable to unsuspecting onlookers.
I would definitely avoid using exposed/visible pins or clips to fix it there. Anything hidden is good, because if you get pulled over you can just open the bonnet NORMALLY and not get defected (doesnt matter how legal it is, if the cop is having a bad day he will defect it anyway).
You can use pins in the original subframe front mounts.... weld or bolt them to the guards pointing back to the firewall, and have them push thru the subframe holes so you can insert the pin from behind the guard, near the tie rod mount..
You can also do some horizontal pins on the inner guard, but the more there are, the harder it will be to get them all in the right spot when trying to put the guards on the car every time.
If you fit wheel arches, maybe have a clip hidden behind it at the seam area. Something on the inside like a right angled bracket that mates up with another one on the A-panel, each with a hole in the centre to put a spring loaded clevis pin thru.
^^when I say 'spring loaded clevis pin', i mean these little things they use on sail boats that hold the mast cables. Beaut little things that can release quickly when you squeeze them and flip a cam on the end. See the pin in the top fitting of this picture....
(not the best picture, but I cant find any others on the net)
You just need to imagine that the pin is holding 2 individual faces together...
When you push the spring end inwards, you can push the other end out far enough to turn the elongated pin 90 degrees so that it can slide back thru the mounting holes. When the clip is in use, the spring holds pressure against the turned pin and holds the whole lot fairly tight, but with enough 'give' to not be a completely solid mount (which is good if the front of the car flexes a bit).