Offset shackles
Well I don't have a dog in the hunt other than to say I got into this same argument years ago and I was hung up on what MP ADVERTISED the offset kit as giving, which at that time was actually .8", not 3/4", so they advertised even more than 3/4" and 3/4" is just not there. It's more like 1/2" as previously mentioned. BUT I'll not speak for every single car, because we all know how all these cars differ from each other a little here and there, so who knows?
For the MP kit, it doesn't really matter a ton how the cars are different. The front hanger is built with a 1/2" offset compared to stock, it's easily measured. The offset shackles in the MP kit are offset 3/4". Even if you weld the spring perches at a 3/4" offset, the front hanger is still only a 1/2" offset from stock, so you're not going to get 3/4". 5/8" maybe, but you're still putting the spring in a bind. Will it cause some catastrophic problem? No, but it's better if everything lines up.
The fact that there's only a 1/2" between the hanger and the frame rail to begin with is just icing on the cake, you can't put a 3/4" offset on the front hanger and not have it inside the frame rail. Now, is every frame rail perfectly aligned with the factory spec? Of course not. But if you're MP, you can't build hangers to sell based on the anomalies. And even on the cars that aren't perfect, well, they're also not usually the same side to side.
Maybe Bob bought a custom set of 3/4" offset hangers and both he the guy he bought them from happen to have cars with frame rails that are 1/4" further in on
both sides. Unlikely, because that's well out of even factory tolerance, but never say never with Ma Mopar right? But that's not what Bob said to begin with, he said he had MP hangers.
Regardless, the MP 3/4" offset kit has hangers that are only offset a 1/2". If Bob would like to measure his spring to frame distance to show his springs really are offset 3/4" and prove me wrong, I'd love to see the pictures.