35chev,I did what joe and joe said.I must have measured everything at least 1000 times,i wanted to make sure it was right.I purchased a rear axel kit that come with new perches.for my application"classic performance parts".I had a problem with there ubolts and nuts,they did'nt fit right.So i had a local spring shop make my ubolts and got the thick nuts,instead of the puinny nuts cpp offered me.I did get reimbursed for the u-bolts"that's a whole other story".
This kit allowed my axel to be center of the rear fenders instead of being forward like original"for my application".You can easily cut your perches off with a thin grinding wheel in a die grinder.I tacked mine after setting the pinion angle.Then i had a friend that is a ace welder,weld them.As a matter of fact,i still owe him a thirty pack of BL.I better get it to him while i'm thinking about it.Take the time and measure and remeasure.Make sure you have it centered on the springs and straight.I used one of those magnetic angle finders.Any hardware store or harbor freight,cummins tool will have them.
When you measure for the drive shaft,you must have the weight of the vehicle on the rear springs.I set jack stands under my rear axel and let the truck down on them.You must also have the transmission in for the driveshaft measurments.
No,you don't have to take the whole rearend apart.Just cut the old perches off,then prepare for the new ones.
Here are a couple of links that i read before tackling mine.I also read every article on this site related to it.It's all great information,read it then reread it.
http://www.carcraft.com/howto/91758/ http://www.streetissuecustoms.com/pinionangle.html Hope this helps,good luck !!!