To install a large hole upper on a small front push pin lower you need an "offset" bushing.
Seems Bushmaster sells them, and probably other AR parts places, or maybe someone here can come up with one for you. A bit of lock tight will keep the bushing in place in the upper, but not needed.
As for the A1 sight, I can't find a diagram.
But it's a lot like the way the peep sight fits into the A2 elevation assembly.
This is off the top of my head, to might not be 100%
Lay the flat spring in from the top, it will fit into a shape in the receiver.
The arch should lay from front to back, not side to side (if that makes sense)
Run the screw in from the left and thread into the peep. Add a touch of oil/clp to the threads.
Making sure the "L" on the peep sight faces to the rear of the rifle.
Helps to hold the peep down against the flat spring during this procedure.
So the screw doesn't bind or drag on the receiver.
Finish threading the screw into the peep sight until the peep is about half way between the ears of the receiver while the screw head is pressed to the left side of the receiver.
Right side of the receiver there's a little hole next to the larger hole that the screw will be sticking out off. This is were the small coil spring and detent go. Spring first, then detent.
These can be lose easy, so keep an eye on them.
Or try and work somewhere were you can find em should they get away.
Now thread/spin the "windage drum" onto the screw.
Here you'll need to keep the spring and detent in place while doing this.
A small punch or A1 sight tool can help depress the detent towards the end.
Here everything needs to "come together", never twist hard on the screw.
Everything should fit nicely.
The drum has a small hole on it's axis that needs to align with the hole in the screw.
once aligned, the roll pin is driven through the assembly.
Leave a bit of side to side play, you don't want the drum/screw so tight they bind.