The whole assembly unbolts from the Jag with just a few bolts but has a lot of brackets that have to be cut off. This is the one that belongs to a member on here.
This is one I found on the net. It may or may not belong to a member.
Another net find with a GMC frame.
The one I have that I bought in Bend Oregon a few years ago compared to a chevy truck front axle.
A closer idea of mine giving you and idea of where you might notch it.
If the Jag is still complete with engine and trans get it on a flat surface and get under the front and measure from the bottom of the crossmember to the floor. When I started figuring out how I was going to set the chassis up so it was at the ride height I want I would block it up that distance off the garage floor and set the front of the frame on it to see how high the running boards are.
A Jag weighs 4000 lbs if you average out several different weights. I can't find a weight bias but the six weighs about 750 lbs. Less weight means that the rig will sit higher with the same springs though.