[dsraven]

it looks like whoever did the clip spent time to get it to fit well. dunno about the ride height etc but it looks like it is done right from what I can see on the pics.
before you spend money on it you may wanna check the frame for straight to ensure the clip was welded in parallel to the chassis.

-start by doing a corner to corner measurement using a steel measure and ensuring your helper holds his end the same on both sides, or use a small clamp to hold the measure if doing it alone but ensure it is the same on both sides. the dimensions can be out about 1/8" according to the gm assembly manual. you can also cross check in an X pattern from front cab mounts to rear cab mounts etc. just make sure you have a good frame before going too deep. judging by the look of what you have I would hope the guy who did the work also did this step first.

-mark each cross member at it's midpoint from side to side, including the clip
-using a string line or a laser run a line down the middle, fore to aft, along those marks, starting at the rearmost and ending at the frontmost mark.
check to see if the string or laser hits the mark on each of those center spots you previously made on the cross members

-if they don't all line up then there is an issue. if they would all line up if you were to move the clip mark over a bit then your clip may be in out of alignment with the rest of the frame. you'll figure it out.

-also check for frame sag. these frames are dead straight except for the kick up over the rear axle and the drop at the front for the bumper (cut off on your frame obviously)

-if it is all good then knock yourself out going forward.
up front:
check the ball joints (gotta unload the spring pressure from them so they can be checked properly).
check the tie rods.
check the idler and pitman arms.
check the center link
check the steering box pitman shaft bearing and gear adjustment
check the control arm bushings
check the suspension snubbers
check the brake hoses and steel lines
check the wheel bearings and seals
check the brake parts, pads calipers etc (I know partially missing)
check the stabilizer bar bushings and links
out back:
check the spring bushings and hangers
check the axle housing mounts on the springs and U bolts
check the wheel bearings and seals
check the diff vent and hose
check the diff fluid level and for any leaks at cover gasket or pinion seal
check the pinion bearing and yoke for wear
check the brake parts (I know, missing)
judging by the pic you may want to eventually partially dissassemble the diff, flush it all out with solvent and a garden sprayer, check all the bearings and seals (replace the axle seals and bearings and check the axles where the bearings seat) and then reassemble just for peace of mind. the diff vent was missing so you have no idea how much crud went down the hole. worst case scenario the bearing kits for a rebuild are actually quite affordable. you could leave the pinion shaft in place for a partial dissassembly. mark the bearing caps so they go back in the same side and the same way up. same for the carrier bearings and any shims for side to side match.

-if you remove the front springs and shocks and reassemble you can install threaded rod in the shock holes and screw it down to adjust the ride height so your lower control arms are parallel to the ground, side to side. that should be your ride height of the frame with the weight of everything on it when completely finished. then your front clip will have the proper geometry to work as it was made. you can then adjust the rear suspension using a home made adjustable link, where the shocks would go, so you get the control arms of the front clip sitting at the correct angle. then that will be your rake angle for the frame going forward. then check above your diff axle tubes to ensure you have at least 3" to work with. then you will know if you need a C notch or not. you can assemble everything with those rods in place and then when the mock up is done and you want to put springs back in up front you can get the proper spring to achieve that same ride height. out back you can adjust the axle housing on the springs with the correct block or hanger length as required. if the truck weighs less than the car did and it ends up sitting too high then you could look for a set of front springs from, say, a 6 cyl car. if it weighs more and sits too low then look for a set of springs from a big block car. or check a spring shop cross reference to find a set of springs that will be correct for your weight. you could also just ask the guys on this forum who have done the same thing. a real wealth of knowledge exists right here.