OK, heres the scoop on pricing of just the center sections with SG units. I have sold more than a few over the years and I have also seen various pricing, mostly way over priced. My general rule is this...
3:23SG=$300.00
3:55SG=$350.00
3:91SG=$400.00
4:10SG=$450.00
Usually anything over a 4:10 or under a 3:23 is sold to a limited market and finding them is not as common, so I am quoting prices for the most common ratios. Also, most people tend to want the 489 case or 742, although there is nothing wrong with the 741, it doesn't sell as quickly as the larger pinion shafted models. The 742 case is my personal fav and I have them in all my cars.
There is also a mis-nomer that the "cone" style SureGrip unit is inferior and not rebuildable, but that is simply no the case. By design, the cone style unit is considered a superior design due to the clutch design being just that...clutches that can slip. The cone style uses a centrifugal outward pressure on the case itself, and as long as the unit is functioning properly, slipping is all but eliminated. The cone style is also rebuildable, but takes more than just replacing clutches and discs, but it can be done easily with just minor machining, provided that the case itself is not scored.
I personally have all clutch style 742's in all my cars, and seek out these because I have them and extra gearsets for them, so I can interchange units and parts if desired, but any 8 3/4" is fine IMO!! Good luck with your search, Geof