Wanna Keep 6 lugs? If so, 10 bolt front is as good as you'll get, but you can get a C14 semi-float rear with 6 lugs. Look in later model light duty 3/4 tons and HD 1/2 tons.
Getting new wheels? Considering 8 lugs? 3/4 ton will get you a C14 full float rear (close to unbreakable) and better brakes on the front, but you still get a D44 or C10 up front. Of course, the ultimate is 1 ton (single rear wheel) which gets you a Dana 60 front, but your not likely to find that in a junkyard...
As for EFI, converting to TBI is not hard with all junkyard parts. If you seriously consider it, get
this book. BlazinOr (now SubOR or something?) put me onto it and it's been a big help...
You can do what I did, just buy a TBI 350 out of a low mileage wreck (mine was an almost new GM Good Wrench crate motor) along with accessories, wiring (under hood and dash), and computer. Very little more trouble than just pulling and installing a new engine. No brackets to find/modify, no belt/pully problems, no butchered wiring, no problems... Worst part is adapting the fuel system to higher pressure and connecting to the bozo metric couplers on the TBI stuff...