Here is a link to American Muscle and the 3.73 gear set all in one.
Ford Racing 3.73 Mustang Gears and Install Kit 100576 (86-09 V8) - Free Shipping
Get ahold of Alex at AM on this forum and get the discount code so you can save an additional 6%. Good thing about the code he gives you , that is you can reuse it over and over again for every purchase you make with AM.
Here is Alex info to get a hold of him.
Mustang Evolution - View Profile: AMAlexLazarus
I would if I were you before buying that or any gear set is take a look inside your rearend, you may need a single part to fix instead of a whole kit to rebuild. I know you want to go with a different gear all together, but as you said, you were not expecting to do this now. I would not hesitate but to use your car for now. I really doubt that it would break down on you any moment from now. The clunking is most likely the ring and pinion are worn, when you are putting the car in forward drive, the pinion rotates so very slightly till it locks teeth with the ring gear. When new, with brake applied the driveshaft moves about 15-20 degrees and then the pinion has locked horns with the ring gear. But if they wear through normal or hard use, the pinion will turn slightly more as wear continues, which is the clunk you hear. I would bet you can take it easy for a while until you can tackle this. But if you are like me, and something is wrong, regardless how bad, you want it fixed.
I am showing you a view of a carrier with the ring gear mounted to it, with the spider gears inside the carrier, also the pinion gear (snub shaft with cone shape gear at one end). The pinion is the first gear in motion when car is in motion, the pinion then drives the ring gear as the two are mated. That in turn causes the movement to the spider gears and rear axle. The rear 2 axles slip into the rearend housing and then slides into position in the spider gears.