(I am asuming you have a 5.0 mustang with a T-5 tranny) (I found one from -Danddperformance.com,T-5 rebuild kit, it sells for $209. Comes with bearings, races, blocker rings, springs, inserts, shims, thrust washers, and snap rings. Does not contain hard parts like gears, sliders or shafts. You can also go to summit racing and get a Ford racing performance parts T-5 rebuild kit for $249.95-should come with everything like the D&D kit.
Now you will also need tools for this, basic hand tools, snap ring pliers, dial indicater, I think i used a feeler gauge. and a shop press to press the some gears off to get to all the pieces you need to get to, it wont make sence to fix 3rd and 4th when you can also do 1st, 2nd and 5th. The tear down aint to hard, just putting everything backtogether is a little pain if you dont remember where everything go's. Get some specs on the transmissino also and make sure everything is within spec or that tranny will not last to long. if there was a scale from 1-5 (5 being hardest) i would give this a 4. If you do this on your own good luck
***with the grinding issue make sure your clutch is pressed down all the way, that has happend to me lol, and yes it is possably a syncronizer gone bad, lost its friction material and it cant slow the shaft down fast enough, so your slider pushing on that certain gear, gears aint grinding its your syncro and dog-houses (teeth) grinding*** oh i hope that made sence
Great advice nibbie this helps me also. So it's likely from what you're saying that if this install was done right it would fix his issue unless of course the gears themselves were shot. Sounds like a cost effective solution to getting a new or rebuilt tranny. If you get the video it sounds like it will make keeping track of the order of parts coming in and out will be easier. I would just write down the part as it came out in order so that when I went to reinstall it would be the reverse order because putting everything back together in the right order seems like the crucial step.