I think my U-joints are shot
 

At about 40 MPH the vibration starts. It just gets worse as I go faster. By 50 MPH or so the vibration gets so bad, the whole car seems to resonate. I have had a slight vibration for a long time, it got worse when I put in the 3.40 gears, and the past few days it got bad enough that I can't stand to drive on the highway. How can I check to be SURE it's the U-joints? Could it be something else? Any help is greatly appreciated.

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66 GT Fastback 5.0, 5-speed, 3.40 gears, Fuel Injection soon

Hey cool stang, I got a 67 GT Fastback myself. Anyway, you can usually hear the u joints clunk when you accelerate or deaccelerate. You might try going under the car, grabbing the drive shaft and twisting it back and forth to check for abnormal movement in the joints.
Dont forget the obvious things like your tires being out of balance etc. Good Luck

I agree with Jerry, try and twist your driveshaft and observe anything unusual. You might also put it in gear and push on it, alot of times even in drive they'll move some and thunk if the u's are bad. I once thought I had a joint going bad and it turned out to be a plug wire off causing my vibration, just a thought. Good luck. BTW I too have a 67 fastback Jerry, it's an S code GTA.

Check the fan clutch too, if you are running the one that comes with the 5.0... had mine go bad.. thought it was the u-joints too...
Can tell,,,try spinning it with the belt on... should turn...

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84 convt,'95 302, performer
3.55's, underdrives shorties
and a '68 stang that WAS nasty ;-)

Thanks for the help guys. I was given a link to a good site on drivelines. The vibration I am getting sounds just like my pinion angle is way off. Guess it's time to replace those saggy rear springs. Anyone know of a quality set/ good place to purchase?

hehhehmule, Actually I have two 67 fastbacks (yeah I love them). The GT is a K code the other is a very original C code. The S code is a 390, right? Anyway fastbacks rule, later.

Yeah, that's right Jerry, it's the 390. A 67 K-code fastback huh? Damn that's sweet and the C's a 2v right? I've got a 65 solid lifter 271hp in my truck, love that engine. I understand it's real uncommon for a 67, congradulations. I had a C-code years back. Boy those 67 FB lines are cool huh?

hehhehmule, Well the story gets alittle sad. The GT was through a lot of hands (translation used & abused) before I got the car. I never got a chance to experience the original engine. There is a plain jane 289 4 bbl in it now. The C's original engine was lost due to my own stupidity. I worked at a marina many years ago. I replaced the engine and the marina accidentally gave the engine to the junk man. Good news is I'm putting a 460 in the car. Any experience with 460's
As far as FB lines, only two other cars equal or come close - 66 charger & 63 to 67 vett.

Can't say I've ever messed with a 385 series much Jerry. I can tell you a FE 390 is a mighty tight fit in a 67 and I think the 460 is a bit wider. As for your GT, anything with a K on it is worth its weight. Just hold on to it till you can find a K engine and you'll be set. They're great they feel alot like a 5.0 but the sound is so much more visceral.

460's rock.. am putting one in my 68 coupe..can get 500hp out of one with out much problem.. FMS has a 460 short block for about 1500, around 560 hp..cool, huh?

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84 convt,'95 302, performer
3.55's, underdrives shorties
and a '68 stang that WAS nasty ;-)

Dark Knight
Oh yeah! I got my 460 out of a 76 Lincoln. In the process of tearing it down. Hoping for a basic rebuild. not much money. I've read a few articles about the swap. Do you have the motor mounts, trans mount, drive shaft, or headers?