The 1947 - Present Chevrolet & GMC Truck Message Board Network

before we get too deep, we will need to know what your skill level is, what you have for tools and shop, what you want to end up with and how much cashola you plan to loosen up for the build.
if you are doing this on a very tight budget in a dirt field with no power and only a pair of pliers that you believe to be a phillips screwdriver then that is a lot different than if you plan to spend 100k and have everything done for you even though you have a huge quanset with all the tools, a hoist and a crew.
really, if you have a garage and some time,put the frame on some stands, level it up front to rear and side to side, then measure it up.
some modified jack stands that will accept some threaded rod on the top instead of the adjustable part normally found works well. what I did was remove the sliding adjustable part of the jack stand, tack weld a beefy piece of flat stock with a hole drilled to fit the threaded rod, across the top of the jack stand base, tack a nut to the flat bar, then use some threaded rod run down through this with the frame placed on top.use holes drilled in the frame and nuts to secure and lock down. use a jam nut next to the flat bar to lock down the threaded rod once the frame is levelled.
here are a few suggestions once it is levelled up.
-do a thorough cleaning of the whole frame. a pressure washer works good here if you have one
-mark each cross member at it's mid point from side to side. use some painters tape and a fine point pen so you can be accurate
-run a stringline from the mid point of the rearmost cross member to the midpoint of the front cross member. all the midpoint markings o the cross members in between should line up with the string. if you have access to a laser that works great for this. really, the digital levels have come down a lot in price and they usually have a laser built in as well as magnetic bases. this can be very helpful if you plan to do any frame work yourself. a shorter digital level can be placed on a longer straight edge, like a piece of square tube, angle iron or another level if you have one. the old fashioned bubble levels are ok too but I know from experience that they are only as accurate as your eye or the angle you are looking at the bubble from. I placed a 4 ft level across my frame rails and got it good. then placed my digital level on top of that and found out the long level or my eye is not that accurate. when you are talking about being within a couple of points of a degree it pays to spend the few dollars.
anyway, now that you found the frame to be straight down the middle you can check for sag by using the string or level on each frame rail. draw the string up tight and then put a small block,like a piece of 2x4, under the ends so it is spaced up enough to clear any humps or bumps. use pieces cut from the same 2x4 or whatever to avoid discrepancies in different pieces of wood. then take a third block and go along the frame to check if it fits under the string like it should. a gap above the block-between the string and block-would indicate the frame is sagged, and likewise if the block is thicker than the gap between the frame and the string then the frame is humped up in that area.
-after you have found the frame to be non sagged the next check would be a cross check from corner to corner. since you have had a clip welded onto your frame you need to pick 2 identical spots on each frame rail behind the welded on area. then have a friend hold the tape for you on those spots as you measure to the rear corner on the opposite frame rail. it is very important to have the tape held the same for each side because otherwise your dimension will be skewed. they say 1/8" out of square is acceptable but that is for the dimension taken from the front of the frame,not behind the welded on clip. if you have nobody around who can spare a few minutes to give a hand then you can also use a plumb bob and mark the floor directly below the reference points, then do the dimensions yourself using something heavy or maybe tape to hold the measure down
-after you have found your frame to be pristinely straight and square,you should check the rake angle that the front end was set up for. the lower control arm pivot shaft should tell the story here. measure the angle, front to rear, that the control arm is sitting at with the frame sitting level. that will be the rake angle. you wouldn't want to complete a build with a certain ride height and rake angle in mind and then, when all done, take it to an alignment shop who calls to say they can't align it without changing the rake angle of the truck
make a plan, post pics, ask questions, have fun.