The SVO uses 14" long Lincoln arms and Lincoln spindles and front brakes. To use 87-93 spindle on the Lincoln A-arms you have to use a ball joint spacer from Baer Racing or IIRC Steeda sells it.
The Mustang SVO uses the stock Lincoln parts but has a special narrower K-Member so it can use the long control arms but keep close tothe same front track width.
The point of using the 79-87 K-member was that it was narrower than the 89-93 K-member. So the idea was by using the 79-87 K-member with the 3/4" Thunderbird arms you are creating the same suspension effect that Ford was trying to do with the SVO. But it will not be as effective as the SVO because the SVO's A-arms were angled differently. But it is a step in the right direction.
The width of the rear is not a major factor in how the car handles. A wider track will make the car a little more stable in turns but there are other more urgent problems in the Mustang rear suspension that affect handling much more drastically. And having a wider rear end adds tire clearance problems. Not that big a deal on 15" or 16" wheel cars but with 17" and 18" wheel cars can be a real headache. Personally I had 15"x 8" wheels with P215/60-15 tires on rear of my 86 Mustang and the first thing you learn is how to roll the inside lip of the rear tire in before it cuts your sidewalls.
On the rear the main factor effecting the handling is the bind caused by the 4 link suspension setup Ford uses and the ineffective way they mounted the rear swaybar. Normally a swaybar mounts to the body on eiher side and then bolts to the axle tube so that it can have leverage to apply resisting force. The Mustang rear swaybar acts more like a big torsion bar. The best setup is to use a factory rear bar and the Steeda auxilary adustable rear swaybar.
Fords solutio on the SVO was much stiffer lower control arms than stock and much stifer upper control arms. But where Ford got it right was to make stiffer upper arms but use a high durometer rubber bushing. This makes the upper setup stiffer but not to the point that it creates bind and hursts handling.
When using a stock rear setup the most effective setup was tubular lower control arms, teh SVO or FMS uppers, a .79" rear swaybar and a adjustable auxilary swaybar similar to Steeda's.
The best solid axle setup on a Mustang is a Panhard bar and torque arm. Expensive but works so much better that people I have talked to with the setup says its a completely different car, that it doesnt act at all like a Mustang.