I think you are on the right track trying to change the tailshaft angle.  If you can find a set of Metwrenches they will probably allow you to remove that rounded nut.  They grip on the flats not the corners and I've had really good luck removing even rounded fasteners with them.  I even got off a nut I had beat a smaller metric socket onto and rounded it off worse before using the Metwrench.  Not sure if they still sell them new.  They were an infomercial deal 20-30 years ago, but were also sold at Sears and maybe other places.  They really do work.  My 3/8"-10mm one has freed MANY a stuck bleeder screw over the years. 

installed Ron Morris 1/2" lower motor mount kit. Changed my T5Z transmission output shaft from 4.6 degrees down to  1.5 degrees down. with no Pinion shims on rear axle vibration is gone. This setup has minimum clearance for repo cast HIPO exhaust manifolds. 

rear end with no pinion shims .6 degrees down. Meaning that drive shaft flange is up and rear of differential is down. the angles get confusing on paper. measured all my angles on apporpriate u joint with snap ring removed.
measured top of distributor before and after to monti carlo bar and dropped 1/2".
results have only drone from FlowMaster 40 mufflers. Not a problem I grew up with 8 brother and sisters

thanks

Ron Morris was nice too talk with went I called him and asked about his product

Thanks
I have been chasing vibe at 80+ mph for over 25 years.