I have a 69 Mach 1 with a used 70 rear spoiler I installed circa 1982.
It's fiberglass and heavier than the plastic 69 spoiler which was notorious for sagging.
There's a prop rod to hold the trunk lid up, but the lid has always stayed up fine on my car without it.
There are also oval cutouts in the trunk bracing for the spoiler mounting brackets.
69 trunk lids lack these cutouts so the spoiler was simply thru bolted.
I see a lot of 70 mustangs with rear spoilers on eBay lacking the mounting brackets which is a shame since when utilized, they make drilling the holes goof proof.
If you need pics, I can search my backup drive and take some pics of my car.
Since I own a 69, I'm not going to speak to the 70 chin spoiler.
I should also mention you could get the rear spoiler on a 70 sportsroof regardless of what factory built the car.
The rear spoiler would be on a Marti report for your car if it was originally so equipped.