75 duster to 72 demon swap
A '75 is a little harder because of the shock mount bumper brackets. As you already noted, there's only one threaded bumper mount hole and it's in the wrong spot. So, you'll have to figure something out. You could drill completely through the frame rail and use long bolts for the standard brackets, but then you'll have to deal with whatever may be in the way on the other side. And to keep from crushing the frame rail you'd probably want to sleeve the holes. So it's not a super easy deal. Personally, what I would do it use a hole saw. Locate the bumper mount where you want it, mark the rail, and drill small pilot holes. Then use a hole saw of a larger diameter than the nuts you're going to use. Take the plug you get out of the hole saw, drill the pilot hole out to the right diameter, and then weld the nut to the back. Then weld the plugs back into the frame rail. It's either something like that or you'll have to open up the end of the rails and work inside of them, but that won't be easy either.
Here's your problem-
This is a standard frame rail up close ('71)
This is a mid to late model year '74, which had all 3 because at some point in December '73 they stopped using shock mount front bumpers on the '74s and went back to standard
So at any rate, you need to add the mounting points for a standard bumper bracket if you're doing a '75. Same thing in the rear. One of the mounting locations for the standard rear brackets should be there already, but the other one is in the middle of the hole where the shock mount passes through the rear crossmember.
I converted '74 over, all the work I did is in my build thread. The initial rear bumper conversion is on page 3. The tail panel swap is on page 6, and then my revisit on the tail panel and rear bumper is on page 9 (I tidied it up and made it look more "stock").
My "new" '74 Duster- or why I need a project like a hole in the head
I did the tail panel swap as well, with info in my build thread. But there's also a "how to" here
Duster to Demon Tail Panel Swap
A '75 is a little harder because of the shock mount bumper brackets. As you already noted, there's only one threaded bumper mount hole and it's in the wrong spot. So, you'll have to figure something out. You could drill completely through the frame rail and use long bolts for the standard brackets, but then you'll have to deal with whatever may be in the way on the other side. And to keep from crushing the frame rail you'd probably want to sleeve the holes. So it's not a super easy deal. Personally, what I would do it use a hole saw. Locate the bumper mount where you want it, mark the rail, and drill small pilot holes. Then use a hole saw of a larger diameter than the nuts you're going to use. Take the plug you get out of the hole saw, drill the pilot hole out to the right diameter, and then weld the nut to the back. Then weld the plugs back into the frame rail. It's either something like that or you'll have to open up the end of the rails and work inside of them, but that won't be easy either.
Here's your problem-
This is a standard frame rail up close ('71)
This is a mid to late model year '74, which had all 3 because at some point in December '73 they stopped using shock mount front bumpers on the '74s and went back to standard
So at any rate, you need to add the mounting points for a standard bumper bracket if you're doing a '75. Same thing in the rear. One of the mounting locations for the standard rear brackets should be there already, but the other one is in the middle of the hole where the shock mount passes through the rear crossmember.
I converted '74 over, all the work I did is in my build thread. The initial rear bumper conversion is on page 3. The tail panel swap is on page 6, and then my revisit on the tail panel and rear bumper is on page 9 (I tidied it up and made it look more "stock").
My "new" '74 Duster- or why I need a project like a hole in the head
I did the tail panel swap as well, with info in my build thread. But there's also a "how to" here
Duster to Demon Tail Panel Swap
