Ok guys i put in the 3:73s and about 75-80 it starts vibrating and gets worse faster you go? I like to cruize on the high way at 70-75 then when i have to make a move then it gets bad vibrations. I am hoping is that the 305 cant push it that far and hoping when i put the LT1 in it will go away to over 100. Or is my gears messed up?
thanks
kevin

You have something messed up man. Who installed the gears? Do you know what all was done? Changing the engine will have zero effect on the rear end vibration.

Was the rearend taken out to change the gears or was it done while attached to the car.

The gears was just done in the car. Nothing was removed or nothing.

common problem when upgrading to higher numerical gear sets as the drive shaft from the factory was poorly ballanced. have your local driveline shop balance it and you should be good!

Was the driveshaft removed during the gear swap?
Was it just a ring gear change or both ring and pinion?

The drive shaft was removed they put new end things i forget wat their called the u joint! then eveything but the axels were replaced.
kevin

The driveshaft may need to be reseated into the pinion yoke.
Often when you get a vibration right after having removed the driveshaft that's what it is. Sometimes it's just a little off-center when it gets bolted back up to the yoke.

I hope thats it! that shouldnt cost a hole lot? Who can fix it?
kevin

will pep boys or someone do it?

You should be able to do it with a small crecent wrench.
Take a long look at it. If it's off-center you should be able to see it.