The Mustang has no frame under it.  Its a unibody car, but plenty of unitbody cars have full frame rails.  The Mustang has a front frame rail and a rear frame rail and floorpan in between.  Weak sauce.  Its just not a good design from a strength and handling standpoint.  Subframe connectors complete the frame rails.  This makes handling more precise and improve traction.  A stiffer chassis allows you to run a lighter spring and still get the same handling. 

There are a bunch of different manufacturers.  Personally I wouldn't waste $1 on bolt in connectors.  Get a good set of weld in connectors.  I have a set from Fatman Fabrication in my '67.  Good quality, fit well. 

Typically they either slide inside the front frame rails or craddle them and then tie into the rear around the front spring mount.  You don't need to strip the car down to install them.  Pull the seats and sill plates and fold the carpet back at most.  Even that's just precautionary.  The areas where you weld aren't right against the carpet. 
 

I have the weld in Tinman in mine I like them

Thanks guys! Can you share any photos of the connectors welded in and a link on where to source them from?
What's the opinion on whether these strengthen up the car to make an outcome better in the chance of an impact?

I have weld-in Tin Man Subframe Connectors, purchased from Summit.  
IMO - I agree w/TKO's post.  They are definitely 'better' than not having them, especially as these cars are older and likely any flexing through the years has weakened the overall structure of the car.  

Good info above. I'm struggling finding my pics or MS's pics of his SFCs, which look great.

I don't know anyone who regrets installing SFCs. Pretty sure it was the first mod I did to my 69. I have welded Competition Engineering SFCs, which only contact the ends of the front and rear factory frame rails.

If you're considering adding them to your 65, I gotta ask if you have torque boxes on it yet. I'd think torque boxes would be my first fix, then SFCs. Torque boxes by definition resist torque, which is mighty important. And SFCs resist linear forces.

I have a web page showing you how to make your own

https://dazecars.com/dazed/sub-frame.html



 

Rudi wrote:

Convertible inner rockers are also a very good stiffening addition to coops and rapid posteriors,
Not really an option for a finished interior though

Fair enough... Mine is finished and well and truly on the road, but thanks anyway.

BobE wrote:

I have weld-in Tin Man Subframe Connectors, purchased from Summit.  
IMO - I agree w/TKO's post.  They are definitely 'better' than not having them, especially as these cars are older and likely any flexing through the years has weakened the overall structure of the car.  

I definitely see your point!

Toploader wrote:

Would it be correct to assume that fitting subframe connectors would be about a 1/2 day job?

 
Mustang Steve’s Law has to be applied to the job or any other modification as well!🤬

Post #3 and #4

https://fyi.boardhost.com/viewtopic.php?id=10979

Suggestion - paint (or coat with something to prevent rusting) at least the top of the subframe connectors prior to installation.  While there is some room to do so after installation, it gets complicated, and one can't see if you've gotten full coverage. 

Rudi wrote:

Toploader wrote:

Would it be correct to assume that fitting subframe connectors would be about a 1/2 day job?

 
Mustang Steve’s Law has to be applied to the job or any other modification as well!🤬

Yeah... I hear you! 😬