Rear
First thing to note is that your rear suspension may be ok without modification.  Check the table below to see the ratings for the part numbers.  Spring part numbers can be seen without removing anything but the rear wheel.  An example is shown in the figure.
 
 

Front 
The front suspension of a Mustang with a 6 cylinder needs help!  According to Dave Schjeldahl, a 200ci. 6 cylinder long-block weighs about 385lbs. However, a 351-C weighs about 550 lbs. Obviously, the front suspension must be improved when installing a 351-C (or any V-8). The front suspension on my '67 was completely shot anyway. Take a look at one of my tie-rod ends. The boot was split and the joint itself is full of rust. No wonder it was difficult to steer.

I bought a suspension kit from Mustangs Plus.  It is aftermarket, but is the best price I could find.  It is possible to replace the bushings and ball joints without replacing the control arms, but it is a lot of work.  You have to drill out the rivets for the upper ball joints and the lower have to be pressed at a machine shop.  The bushings for the upper and lower arms also need to be pressed out and new ones pressed in.  In San Jose, CA area, machinists charge about $10 per bushing....I just bought new arms. So here are the parts. The brake rotors, spindles, etc. are used, as is the anti-sway bar. The rest is new from Mustangs Plus.

These close ups show the new heavy duty coil springs compared to the originals, and the "new" 15/16" sway bar compared to the original. The swaybar mount points are exactly the same as if you had this bar installen in 1967 as orininal equipment. So, I just bought new bushings for a '67 Mustang. I installed poly-eurathane bushings throughout the front end. I like the feel, but other people (on internet sites) claim eurathane is too stiff for their taste.Because the new springs are "stronger" (have a higher spring constant) they are also shorter, giving the car a lowered front end with a stiffer suspension. With the new front-end parts and the stronger (front) anti-sway bar, I really don't need power steering and it is extremely stable (flat) through the corners.

Here are some pictures of the restoration process. First thing to note is that the inner fenders were both cracked. Since this is an integral part of the suspension system, they need to be fixed. Otherwise, the fatigue cracks will continue to propagate further. These are easily welded with the 6 cyl. engine in place, so I drove it to a Maier Racing in San Lorenzo, CA, to get it welded. If you wait until after the V-8 is in place, you can't get access to the cracks from the engine compartment. These pics show the size and location of the cracks.Click on the pics to get a better view.

I couldn't get an adequate spring compressor, so I used hydraulic jacks. This is very dangerous. I tied the springs with a seat belt si if things slipped they wouldn't go flying far.

To see the rest, go to the next page.