I would suggest that you use the 11&3/4" rotors & the required adapters(I'd use sliders) & some better pads & follow a bed in procedure. the larger ones are not too $$$ nor hard to come by & make a big difference in what you end with for braking. If not as said the 73-76 A body & 73-74 E body 10.87" rotors/adapters are what you need to find. EDIT the 76 (only) A body calipers have about a 5/32" larger bore than the 73-75 calipers which gives you better brake bias. The external dimentions are the same as is the price.
For a '68 B body car you probably want to hang the calipers to the rear. In that case I'd use the late model FMJ calipers since they are designed to be rear hung. They have the inlet and bleeds in the correct location to be rear hung. Then you'll need to have a set of custom brakes hoses built and you'll be all set.
I run the 73 Up A body spindles and 11.75" Cordoba rotors with AndyF's Wilwood adaptors and the the Late model Aluminum M/C....works phenominally.
I'd like to know if there is really any weight savings though...the WW calipers are super light but those big rotors seemed about as heavy as the drums and backing plates that were on there. With the oem calipers I'd almost bet it's heavier overall.