[dsraven]

my advice,
-decide on a rake angle and ride height
-decide the tire size (diameter) for the truck, front and rear. do engine rpm calculations with diff ratio and trans (overdrive or not) of your choice and decide what you want there as this could have an effect on the tire size that best suits your needs
-set frame on stands at rake angle you decided on. level side to side, do frame checks etc
-decide on where you want your axle center line with regards to the "look" of the tire in the wheel opening for the ride height you like
-mock it up with tack welds and set suspension how it will sit at ride height. use threaded rod to simulate a spring so the ride height can be held where you want/need it. this keeps the control arms where they need to be for geometry to work properly, then install the tires. do you still like the look?
-if you like the look then do the weld in. if you don't like the look do the required changes until you like the look, then do the weld in. trial and error. mock up the tire in the rear wheel opening to match your desired ride height and rake angle. that will give some indication of how much the rear axle will need to move up or down.
-assemble the IFS for real. do mock up alignment
-install complete drive train (possibly minus driveshaft). if this is not do-able then simulate the weight in the right areas so the truck will have the finished weight sitting on the wheels. install mock up weight of entire truck components including a full tank of fuel, seats, rad, battery, spare tire etc etc
-set truck down on it's wheels and recheck the ride height and rake angle. if the rear of the truck is not where you want it for rake angle then do what it takes to get it where you want it. springs, tire sizes, spring hangers etc. remember to keep enough room above rear axle so the axle will not bottom out on bumps. a good suspension snubber is an asset here. if a C notch is needed then you start down that road. you need to have at least 3" above the rear axle at ride height and even that will bottom occasionally. a good snubber, like a urethane bee's nest style used on newer trucks, would be an idea.

my original truck was going to be air ride suspension so totally adjustable from a mock up perspective. with coils up front and leafs in the rear you will need to get the weight of the truck on the suspension and allow it to sit for a bit so the springs settle out. the front end is mock up-able (?) because you know where you want it to be. the variable is the rear suspension ride height and that can be guessed at but you won't really know the actual rear bumper height of your truck until you load your truck's stuff onto your truck. there are a lot of things that can change that in comparing to another truck like yours. new parts, used parts, different manufacturers using different leaf counts or whatever. the important thing is to know what you want to start with, set up the front end for that ride height and rake angle, then adjust the rear AFTER until you get what you want. otherwise it's a guess. an educated guess, but still a guess. the front end is a weld in and has certain geometry rules to follow. the rear end is something you can change when the front end is done.
rear end too high, prolly gonna be the case, you can change axle to be above the springs, change spring shackles to different length or change the hanger for the shackles to be in a different location. you can change the forward spring hanger mount locations on the frame or change the design of the spring hanger to incorporate a different spring pin height location with relation to the frame. do driveline angle adjustments accordingly after you get it figured out. angle of rear axle pinion u joint needs to match (but be opposite) the angle exiting the trans.