Hi guys. Just want to extend a thanks out there for all the great information. What a great site!
I am looking at purchasing a 8.5" 10 bolt, with a 3.73 gear ratio for my 68. My 68 is a mono leaf, and this unit is from a multi-leaf car. I guess I have 2 options. 1) buy a set of multi-leaf springs, and if I do that what and wear do I start to look for these? 2) From what I understand the mono leafs are narrower, so is there some sort of pre-fab adaptor out there to mount on the differentials saddle to handle this type of conversion?

Thanks
Ken

I switched my 68 Camaro from mono to multi using the National Parts Depot package.

get a set of $20 Mopar spring perches and weld them on in place of the stock perches, and just use the stock lower mounting plates.
it works- i did it on my nova.

I made the spacer blocks for my cars in the past, out of aluminum bar stock.
If you place the spacers on top of the spring, you will lower the ride hieght about an inch and a half.
If you place the spacers below the spring, it will not change the ride height.
Placing the spacer below the spring requires a socket for the spring pin and a locator pin in the bottom of the spacer for the sring plate.

I've also done spacer blocks for this change, exactly as detailed above.
You csn buy "lowering blocks" from summit and use those, get em 1" thick.

I made the spacer blocks for my cars in the past, out of aluminum bar stock.
If you place the spacers on top of the spring, you will lower the ride hieght about an inch and a half.
If you place the spacers below the spring, it will not change the ride height.
Placing the spacer below the spring requires a socket for the spring pin and a locator pin in the bottom of the spacer for the sring plate.


You can buy these spacer blocks with the pins and alignment holes like L72 mentions ready to go from www.Speedwaymotors.com (http://www.Speedwaymotors.com). Just search using the keyword "lowering block" and you'll find the extruded aluminum lowering blocks. And don't use Beltech (sold by Summit) because they were too narrow when I tried them on my car. They're designed more to be used with S10 pickups which have narrower leafs.

And like deerhunter said, if I were you, since it's so easy, I would remove the mono-leafs and just replace them with multi-leafs. Replacement springs are about $250 last I checked...

I like option #1.

When I installed the 8.5 rear in my Camaro I also replaced the sagging mono leaf springs with new 4 leaf springs. I purchased the springs from Tamraz on eBay (http://cgi.ebay.com/ebaymotors/1967-81-Camaro-Firebird-4-Leaf-Rear-Springs-Multi-leaf_W0QQcmdZViewItemQQcategoryZ33584QQihZ016QQite mZ260081557652QQrdZ1). The new springs and Energy Suspension poly pads fit the spring perches on the 8.5 perfectly. Reproduction multi leaf shock plates and new shackle bushings will also be needed.
8.5 Swap Pictures (http://geocities.com/sdampier@sbcglobal.net/Team_Camaro/Rear)

Thanks for all the great advice. I think I'll go the route of buying new leaf springs, plates and related conversion hardware. Seems like a simpler route to go.