Crossmember SAG is what its called...
it comes from the leverage of the suspension pushing in on the top of the frame rail until the top is narrower than it should be... it was horrible in my 66 GTO.. i actually went to offset steel bushings produced by dick gulstrand..
years later.. i met a frame repair guru.. he showed me how to fix it.. i had him fix several of my cars..
i actually have done alignments as a tech back in the early 80s.. and actually have my own bender caster camber gauge and turn table plates.. i used to have a 4" W beam about 5 feet long.. it was leaning against the door frame in a former shop. i think a scrap guy took it.. W beams have wider top and bottom flanges than I beams. a 20 ton bottle jack.. or two.. some 4 or 6" square 1/2" thick plates to spread the load of the bottle jack top.. 2 sections of frame pulling chains with hooks on at least one end..
first step... lay fender covers across the fenders.. lay a 2x4 across the fenders directly over the upper control arms.. clamp or mount some fairly thick wire.. or welding rod... so you can measure around the motor and to the inside of the upper control arm mounting brackets.. outside of the shim packs if there are any.. 31 1/16" between the inside faces of the upper control arm mounting brackets..
if you have to bend the welding rod to get around the obstructions.. then carefully lift it off and touch a pre measured distance on the floor.
setting up... it really helps to have turn tables and wheel alignment gauges.. and a perfectly LEVEL place to work with.. you need to set both upper control arms with a minimum of 2 1/8" shims.. plus what ever you need to reach your caster.. even if the camber is way crazy..
make sure your car is level.. let a level across the core support or the cowl.
write down on the floor on each side... the exact measurements you get.. caster and camber.. jack front of the car up so there is at least 10 inches under the front tires.
check the alignment again.. make sure that you have jacked the car up and supported the frame at the kickouts under the passenger and drivers feet level. .. yes.. with the wheels dangling.. write the camber number next to the on the ground camber number. calculate the difference.. then figure out how much more you need to add to the first number to bring it to 1/2 degree camber positive left side and 1/4 positive camber right..
now.. remove the front tires.. loop the frame pulling chain middle behind the upper control arm shaft .. not pulling on the upper control arm. but pulling on the shaft and bracket.. hook the chain in a loop just off the ground.. you may need to unbolt the brake line brackets and move them slightly so you don't crush them..
slide the W beam thru both looped chains so its several inches off the ground. set the 20 ton bottle jack on top of the W beam.. directly under the cross member. place one of the square jacking plates above it..
look at the numbers you have calculated.. how far do you need to go.. which side gets more.. if you need to go more on one side.. put the jack slightly toward that side.. it will exert more force on that side.
you can start jacking.. doing it a little at a time watching the camber change on gauges.. warning it will spring back.. you don't want to go too far..
what are we doing.. put your arms straight out to the side from your shoulders.. bend your elbows so your forearms point straight up like you were flexing your arm muscles. now.. move your wrists outward slightly.. that is what you are doing.. spreading the top of the frame..
things you don't want to do.. you don't want to get the chains behind the upper control arms.. or over them.. if you pull on the arms at all.. you are going to be pulling really hard down.. you will usually pop the lower ball joint out of the lower control arm. i have seen it done..
you can.. use all thread with stacks of various sized washers thru the shock hole to hold the suspension at ride height.. while its up in the air.. gives you a better idea of what you are doing.
if you don't have wheel alignment turn tables.. you can make your own..
1 in. Roller Ball Bearing mount some of these to the bottom of a piece of 3/4" plywood.. if you have a perfectly smooth concrete floor without paint.. they might ride directly on the floor.. if you use some 12 gauge steel sheet metal.. they will roll really easily on it. allow the tires to move in and out easily.. turn left and right effortlessly.. you do have to build plywood spacers to go under the rear tires that match these fronts in height.. do them in a circle pattern.. use at least 8 on each piece.. there are 5/8 versions but they hold less weight.. so it would take more to support your car.
more measurements.
not nearly as important.. but worth checking..
between the inside of the frame rails below the lower steering box mounting bolt and where the idler arm bracket mounts. 28 1/8"
outside of the frame rails where the front bumber brackets bolt on.. 35 1/2"
i don't know if these grade 43 chains are strong enough..
3/8 in. x 14 ft. Grade 43 Towing Chain it will take 2..
i think my frame pulling chains were grade 70 3/8"
i hope this helps.. i used to do these in my driveway.. probably took me 2 hours to set up and pull them. on a frame rack as my friend(RIP) would let me use his frame rack.. i could do it in 45 minutes.
2 to 2.5 degree negative caster... right 1/4 positive camber left 1/2 camber .. depends on the crown on the streets. i would fine tune the front toe in by having a friend drive the car down the block toward me as i worked the tie rod adjusters. so i could get almost perfect toe in. the gto was so perfect.. i could push it easily with one hand when i was done.. i have seen small cars that took many times the force to push..