How many horses?
 

with a new 289 bored 30 over and stock except 600 cfm edelbrock performer -- edelbrock performer 289 intake - 302 heads-stock or very mild cam- tri Y headders and 2 1-2 exhaust with 2 chamber flows. the engine is bran new..nothing referbished all new parts. also the previous owner messed with the rear end gears alot and i dont know what gears are in there but it has enough torq off the line to snap your neck. also when im just cruizing at about 15 or 20 mph i hear a sloshing sound from the gears...

With some tuning I would say approx 300 since the 289 Hi-Po had 271 stock.

Sloshing in the gears? I would have to say it's probably sloshing of the gas in the gas tank. I don't have carpet or anything in my trunk and I can hear the gas sloshing. Who knows. Good luck it.

im sure its not the gas tank becuasei can hear it even when im pratically running on fumes. the sounds isnt really a slosh more then its a repeating sound depending on speed. at speeds above about 40 the sound goes away. Also was wondering if 66 mustangs came with 8.8 or 9' rears. and if they have open diffs or if they are limited slip.. ..im sure that mine is a limited slip.. also i know that the guy before me who messed with the gears whatever he did. did not have a clue in hell what he was doing becuase when i check to see what was wrong with the gears and just replaced the gear old i fould regular black as crap engine oil in it. so i took that out a put gear oil in.. but it goes to show some people are clueless.

Your 66 came with either an 8" or a 9" rear end. 8.8s came way later. If it's on 8" rear you would be able to get a socket on the bottom most bolts that hold the third member in. If it's a 9" rear, the way it is maid only allows access to those two bolts with an open end wrench. You can also get information from the driver's side door tag and that should tell you numerous things such as whether it's a trac lock etc. One way to tell that is to jack the rear of the car up and support on jack stands. Turn one of the wheels forward manually. If the other turns forward, it's trac lock.
If it turns backward, it is open. The rear end could be an 8.8 but most people transplant a 9" into a classic stang instead of an 8.8
And on the rear end, it sounds as if the gears we're not set up correctly and are making noise. If they are not to worn already, you could pull the third member out and have the lash reset on them by a mechanic. Although it's probably better to just replace the ring and pinion.

Ok, so I need to go back to English class and learn some spelling.

ok iv done that before and they turn opposite directions.. but if i did a burnout then one wheel should spin and the other should not correct? on the way home today i gassed it at a light and i could feel the back set a little and then it did a real nice clean burnout. i looked in the rear view and could see 2 burn marks that are even leanth. i dont know... i just want to save up and take the whole thing apart and start fresh. it also feels like im getting a lot of drag from the back. i can go downhill without my foot on brake or gas and i slow down from 40 to 35 in about 3 or 4 seconds. dosnt seem right..

I'm not sure how fast you should slow down, there are several variables involved in that too, but if your rear end is making noise then it probably has inproper wear and needs some sort of attention. If your rear wheels spin opposite direction, it is an open differential. That is what I have always been told and what I've always went by. If I happen to be wrong, someone will correct me. I can't explane the physics of it but both tires do sometimes spin with open diffs and from my previous experience, usually when you get on it hard. Hope I've been of some help.