i hate to say it because i don't want to come off as an elitest in anyway, but you gotta pay to play with this stuff. solid build so far, don't go selling it short trying to bandaid junkyard parts into something else. sure, those lift springs might be "stiff" on a bronco, but you've probably got close to 1000lbs of extra weight over the front end when you start adding up the diesel, turbo ducting, crew cab, thicker frame etc. that all plays a huge factor in spring rate. and throwing SuperDuty springs in there isn't a great idea IMO either, those trucks are a LOT heavier than the "OBS". when it comes to designing custom suspension setups, the answer is 95% of the time "COIL-OVERS".
you need a 2.5" diameter coilover, anyone telling you a 2.0" will work is lying to you (which is where i'm assuming you got that $700 price). the options afforded to you with coilovers are much better, you get a shock that can be rebuilt forever, easily changed to what ever ride quality you want, and easily sprung for what ever weight you add/subtract in the future. your junkyard springs are going to always be too soft, because they were setup for something completely different, and a 5100 is not enough shock to adequately control the setup either, especially if you are using it to compensate for a spring that is too soft. i would also steer clear of FOA, seen and heard too many stories of their stuff falling apart...like they say "buy cheap, buy twice". PM me if you want to discuss further, I
might work for a company that specializes in what you're doing, but we aren't setup as vendors here yet so I don't want to go ruffling any feathers. At the very least I'd be happy to give you pointers on where to go with shock selection, setups and how to select spring rates.
my buddy has the exact same truck configuration as you, but he kept the TTB and converted to 2.5"x8" travel coilovers; it cycles 10 or 12" at the wheel and is still stock height:
here is my 77 F350 running a similar setup on 2wd beams (throwing a D60 under my truck is now as simple as welding some shock tabs on, changing spring rates and adding a trac bar).
if you rode in either truck you would be blown away how well they ride for what they are, i've ridden in a lot of big dumb trucks and these are both up there in terms of "caddy-like ride from a 3/4-1 ton". the first truck has over 5 years on that setup including several 3k mile towing trips with a 30ft race trailer. and my truck gets used around the house thoroughly and often hauling a dozen rail ties, 2 yds of fill dirt etc.
carry on with the awesome build thread, and for the love of all that is safety, please make the frame side trac bar mount as burly as the axle side