Do a google search for 65 mustang suspension upgrades. There is everything from bolt on parts up through complete welded K-members. probably over 100 different vendors offering shocks, A arms, springs strut braces poly bushing you name it. You'll also get at least that many opinions here!
Especially loud will be the opinions of those who never set up a car for auto cross, or drag racing.
First pick you style of racing you can make an all around car but you will be beat at the 1/4 miles by guys who've set p their cars for weight transfer and max traction in a straight line. the same the other way a car set up for a particular style of racing will perform better in that arena.
Having raced both straight line and mall lot autocross with a 65 that i drove on the street every day, I can tell you my set up.
KYB high pressure shocks at all 4 corners 1 1/8 sway bar on the front 3/4 on the rear. welded shock towers, gussets in the aprons. all poly bussing throughout the front end except the sturt rods. A full monte carlo bar and export brace set up.Antidive upper arms with poly bushing. 67 style lower arms with eccentric adjusters. I added the old two sided torque boxes to my coupe as well as full subframe connectors. I added the convertible style rear reinforcement bar at the taillight panel. As well as convertible springs front and rear. I did not use the shelby drop on the A arms as I wanted a taller "15" wheel and tire set up.
I also changed out the yoke on the trans
for a later style so I could use the bigger u joints. I used an "equalock" posi with the bellvile spring instead of the gear driven Traction lock. A hipo or K servo on the trans
, drive shaft safety loop Inside a dead foot pedal on the left side. 69 high back seats, wider lap belts and a pair of over the seat shoulder straps on the drivers side.
The early chassis were weak and prone to flex under hard usage in the coupe and fastbacks.Lots of guys would use the convertible rockers seat pans and frame extensions to stiffen the cars up.
As far as results HMMM i could do okay in the autocross but int he early 90's Toyotas freaking ruled the ameture circuits in the North east. Drag racing Brackets I was consistent but running in the M class was competing with vettes and early cudas, so not a lot of joy but always made it through to the quarter finals and once or twice to the semi's