Thanks Guys, I measured my spring end bolts and they are 8 inch's long with the nut screwed up to max thread run out.  I ordered a threading die to cut some additional threads to get me closer to the 6-1/4 original equipment bolts. Will cut of the excess.  Thanks again for the help.

You might check the head of the bolt to see if it's a Grade 8 bolt or not. I believe the 6" OEM bolts were Grade 8 for their strength. I mentioned this only because it might be hard to cut threads on it if it is a Grade 8.  

Just my honest opinion,  it may be easier to just buy a grade 8 bolt in the hardware store. They are not that hard to find. Chances are the one in the car may be the same quality, not O.E.M. 

With a little work I was able to extend the threads on the 9/16 diameter grade 8 spring bolts.  Car is all set, was able to get the correct ride height back. Thanks everyone for your help.

👍  👏

Just curious, was your 8" a Grade 8 bolt?

Edit: Oops, didn't read your last post.  😟

In 2021 I rebuilt my 70 LS5 coupe using a TRW mono leaf 340 lb. rear spring from Muskegon brake co. They were very helpful with the purchase.  The responsiveness in the rear has improved a lot. It’s a daily driver with typical curve roads medium speeds in my area of Va.  I noticed the best ride  improvement on the Blue Ridge pkwy last fall.  It did raise my rear height about 1.5 inches, which has put it just above the front height. You can lower the ride height by using longer bolts.  Good luck