super stock spring install question

planning on putting in the restored rear and 1980's nos super stock springs - going in a 69 RR. I bought the SS extentions - my question is are the shackles the same as standard shackles ? Trying to avoid a surprise tomorrow. Using standard shock tower and shocks -

Shackles are the same, but U bolts need to be longer

I bought the U bolts about 10 years ago - I bought the longer ones for the extra leafs. Is that what you are thinking about ? or is there something else I need ? The car is the A 31 package so it had the performance leaf springs to begin with.

You may need shock extensions, iirc I possibly did with my 65 dart with 003/003 SS springs.

instead of shock extensions, i would go with longer truck, or C body shocks. i always thought those extensions looked kinda hoakey. just my opinion.

I doubt for street use you'd need the longer shock, but at the track the longer shock may prevent topping the shocks out, if the chassis hooks and you get some rise out of it.

The only surprise you may get is that the rear end will sit very high. You MUST use extended front shackles (SS springs are the shorter A-body length). Rear shackles are OEM stock. If you decide the rear end is too high, you can use the upper holes in front extended shackles, but that throws off your pinion angle (will need to cut/replace/re-weld spring perches). You can also lower the rear to a "normal level" with lowering blocks (aka "truck lift blocks") and the el-cheapo ones at auto parts stores are pretty darn close to a perfect fit (little massaging of the leaf spring pin and/or lowering block locator hole)

Since you're trying to avoid suprises I'll throw this one out there...Depending on the SS spring arch you may run into a situation where the spring won't reach the shackle because of the increased arch...You may need to use a piece of wood between the frame rail and the rear spring eye to slide the spring twards the shackle as you jack up on the center of the spring to decrease the arch and get it to the point where the shackle will insert into both the frame hole and the spring eye...
After you let the pressure off the spring you'll need to make sure the shackle swings back into position instead of rolling forward as you rest the vehicle's weight on the assembly...

Be sure to check your pinion angle after the SS springs are installed. I used the upper hole in the front hanger and my pinion angle was off by 13 degrees.

Measure the distance form the centering pin to the front spring eye; most if not all SS springs will be 20". Your RR springs are probably 22". That means new front hangers, which AR, Mancini, etc sell.
If you get into some grief getting the springs flat enough to get the rear shackles in the correct position - bottom of shackles pointing rearward - you can use a length of square tubing and a couple large C-clamps to flatten the spring a bit.
R/R of car may sit higher than the other corners: if so, grab a deep 3/4" socket and a long ratchet/breaker bar and raise the L/F using the torsion bar adjusting bolt.
Definitely check pinion angle, and shop around for the shims rather than paying through the nose for the MP shims.
I'd run the C-body shocks (same as D100 fronts) rather than the cheapo extenders.

I was planning on using C body shocks - not because I knew any of this - just because I have a set of NOS Gabriels. Thanks for the tips. The RR is looking great. We didn't put the rear in today - tomorrow - i'll post pics. The car had a 3.91 originally but I went with 3.55. My 68 has a 3.54 dana - and love the feel - so I wanted to get it close to that - saved the 3.91 for the bee.