[dsraven]

you need to know the tire size you will use for the final so you mock it up with that diameter of tire. same for the rear, or at least have the truck sitting on stands at the rake angle you want it to be when done. this is because the cross member needs to be installed level at ride height, fore and aft as well as side to side. the lower control arms need to be level in the same manner at ride height. do a frame check before you start looking for out of square and sag. put the stands under it so it will not teeter totter when the weight of the front suspension is removed and also so you have room to work. use a digital level if possible because they are a lot more accurate than a bubble level.
what suspension kit did you go with? just curious because I think some people get sold a suspension kit with drop spindles and it ends up lower than they thought it would be. a MII is gonna drop the truck some to begin with. it seems like the parts guy has his version of what people need for the look that HE likes and then pushes/sells that version more often than others simply because it's what he likes. then the new buyer gets hooked because it's "what everybody is putting in". well, everybody is putting it in because thats what he sells them. some are not happy with the lowrider stance because it bottoms on every curb and it also involves a "C" notch in the rear suspension and possibly a bed floor height rethink.
anyway, long story short, do some research into what will get you the ride height you want and also think about how you plan to drive the truck and on what kind of roads.
attached are some pics of my TCI cross member and how it I installed it. turned out the crossmember was either cut wrong or was the wrong one for my truck-shipped wrong part. not a bunch of help from the vendor or the supplier so I had to figure it out myself as far as what angles were needed etc. another question to ask from your perspective, before purchase. a dimensional drawing would be good showing pivot point spacing etc, I never got one with mine. pics show stock axle center line and 1" ahead of stock. the tire is a 225/75r15. TCI MII kit with 2" drop spindles. possibly not at the same height as a normal TCI unit because I had to trim the cross member to fit my frame.
when assembling use a long level or straight bar and place across the lower control arm pivot bolts to check for side to side level. don't rely on the cross member being made to exact dimensions. also, if you can, get some long rods the same dimension as your lower control arm pivot bolts so you can assemble the parts using the rods which will be longer than the bolts would be. this will enable you to have the rods sticking out of the holes far enough to place a level on them to check fore to aft level of the cross member. again, don't rely on the manufacturer getting that precise when the steel is folded over to make the box of the cross member. if you haven't bought a kit yet I would seriously look at a cross member kit that has the upper and lower control arms welded onto the member already instead of in separate parts that you need to locate and weld on. that way the dimensional geometry work is already done for you and you simply need to locate the cross member and level it, tack it in, then assemble the parts to a mock up stage and see if you like the look in your fender openings. the pics show the home made tool I used to hold things together during mock up. you can't see it in the pics but there is bracing at the frame behind the front bumper as well as cross bracing under the cab. I also built a "caliper" to check the frame rail width periodically to ensure stuff stayed where it should be. one pic shows the suspension assembled with a magnetic digital level on the caliper. notice the upper control arm angle is upward towards the frame? it should be level or downward slightly. otherwise when the truck goes over a bump the tires will angle outward as they go up until the control arm is level, then angle inward after that. this could cause tires to rub if they are already close to the fenders with the wheel offset choice or axle centerline choice. one pic shows the stock axle centerline and the other is one inch ahead. remember that there is quite a bit of room for adjustment so if the mock up isn't done right the alignment guy may move the axle centerline by doing caster adjustments. thats why it is important to get the angles as close as possible during mock up.
yeah, I know, there are lots of guys out there who have done these with a bubble level in a dark garage and a flux core welder. "they drive just fine". in the end it is up to you but some home work and forethought can go a long way in getting what you want. before welding in do a complete suspension travel check with the tires turned all the way-both ways- just to ensure the tires don't rub on the fenders when wheels are turned and you go over a bump.
tip, center punch a mark on the frame as a reference before the old suspension is taken out so you know where the stock axle sat. usually referenced from the center of the stock front spring pivot ahead of the axle. center punch a mark on both sides so your checking is always from a given point, not an eyeballed point.