Subframe connectors welded in for sure.
IF the ride in your stang is really worn and just plain stinks, consider the Bullitt suspension kit as a "better than stock" upgrade. You'll enjoy the ride so much more. Cost is around $400 IIRC for the kit: springs, shocks, struts, swaybars, possibly endlinks.
A new mid-pipe will improve the sound and add some more power to your car. Deciding between an H or an X is all up to what sound you like best.
If you've got the stock shifter, consider an upgrade like the Pro 5.0. That short shifter has totally improved the fun I have in my car, not to mention never missing a shift.
On the non-performance, but all-important ideas......consider a tune-up, clean the MAF sensor, fix the ground connections (DIY for $20 or less), new brake fluid, and replace other fluids and coolant hoses. How old is the serpentine belt? These aren't sexy mods, but sure make owning the car more enjoyable and reliable. Don't forget to Seafoam your intake/fuel while you're at it.
Since your new, what octane gas are you using? If your motor is stock, don't waste money and power on 91/93 octane.