Cut the old perches off.

Fit the axle with the new perches in place. But just bolted together. The U-bolts will hold the axle in place.

Center the axle under the car. Check the axle end play is adjusted correctly first.

Once centered check your pinion angles. Get the angle the same at the rear UJ as it is at the trans end. You need sommething more that 1.5 degrees and less than 3 degrees (see drawing). Then drop the pinion slightly from that setting. How much you drop it is subject to debate as it depends on how much your pinion rises under load.

Once you have everything lined up just tack weld the perches in place so they stay put.

Now if you can weld OK you can just weld the perches on right there. If you would rather have someone do it for you might have to remove the axle and get it done somewhere.