1969 Mustang coupe

the Tremecs shifter comes up in the exact same place as a four speed. The t-5 comes up a little more forward and alittle farther right (which means you have to cut the floor, and the shifter is further away). My tremec fits a little tight in my 68, and I made a spacer to angle the shifter back about two inches. I could reach it before, but fifth gear was a bit far. Now it's much easier.
I think your car probably weighs about 3300 pound with a 351. An aluminum manifold and heads would take off about 70 pounds or more. Don't get springs retensioned. Springs are only a few hundred dollars, and you won't have to do all the labor again for many years. You can get good after market ones that will lower your car and also prevent axle hop, all in one part (instead of buying several other parts to add on seperately to do the same job).