I have Blupac rear springs which are exactly the same as EFS, just rebranded. I used the top 3 leafs with a flat single leaf out of an LJ80 front pack as the overload. This was fine in an unloaded car and gave a vastly improved ride over my sagged out original springs. I think this worked out to be roughly 1" of lift over the sierras original (non sagged) ride height.
The second time we took the Sierra traveling I added a rear leaf to each side to give it a bit more height/sit level loaded. From memory I used leafs from an original sierra rear pack that I recambered in a press. When traveling it's similar load to yours with the drawers/fridge ect. It's great on high speed gravel and through wombat holes ect but a bit stiff empty.
Originally I used the factory rear shocks but they shortly died. My ultima rear shocks were utterly horrendously stiff. Now it has a pair of factory MQ triton rear shocks which are still on the stiff side but much better than the ultimas. I didn't pick the triton shocks, they just landed in my lap and were the right length/mounts. As part of fitting the long range tank I had to make a shock relocation bar which would reduce the stiffness of the shocks a bit. The Gabriel shocks I have in my LJ are a nice soft valving but that's commodore rears, not the hiace shock. I have no experience with those but I'd be keen to hear your thoughts if you go that way.
Mine flexes pretty well for what it is, it is more compromised towards being a touring vehicle than a weekend fun toy/competition vehicle. It doesn't settle on the rear bumpstops when flexing but it will touch them on rough roads/big bumps. It's fantastic on high speed dirt roads, we cruised at 90km/h on The Gibb and Great Central Road when a lot of other vehicles were reduced to a crawl. You need to have a compromise goal in mind. You can have super soft flexy leafs but it's going to roll around on the road, get bottomed out on big hits and have fairly short spring life. Or you can have machine that takes a load like a champ but punishes you unloaded and flexes like a brick. Like most things, the extreme end of each compromise results in a 1 trick pony.
I've built up a few leaf packs for various vehicles. It really is a trial and error process and often takes a few goes to get right, sometimes a little short leaf on the bottom makes a big difference in ride height. When you bend the spring clamps out to get the leaf pack apart don't bend them back in until you're happy with the leaf otherwise they fatigue. Also some people cut them off, I've done this and I'd only really recommend it if you were chasing every last little bit of travel out of it and weren't concerned about spring life. They really make the springs more reliable (don't splay out) and less likely to bend a main leaf. Overload leafs are important as they prevent/limit axle wrap and thus make the spring last longer, the standard EFS overloads are wayyyyy to thick for a Sierra though.
I'd take the shocks out and go for a drive, if it's still rough as guts you know it's needs the springs derated. If it's smooth then you know you need to chase shocks. Based on my experience the ultimas suck but those leafs will be stiff too.
