What clutch assy are you running ? I have a Center Force Dual Friction clutch in mine and found that the centrifical weights were slipping off to one side at high RPM's and causing a terrible vibration, doing some research I found many complaints of this happening, I removed the weights and that got rid of that vibration, still fighting driveline vibrations tho.

Geez you all live in the tropics, here in South Dakota it was -37 air temp and -58 wind chill, wasn't sure how cold it was by my thermometer since it goes to dashed below -35, National Weather Service recorded the official temps, chilliest spot in the nation, warmer in Antartica. 

Having worked for John Deere for nearly 30 years now I will offer up the way JD recommended to install axle bearings and transmission bearings, it is by heating the bearing to 350 degrees either in oil or a toaster oven, we used a deep fryer for many years until one of the techs spilled hot oil over his hand, luckily he was not burned more than like a slight sunburn, then we went to the toaster oven, granted this will not work for sealed bearings but it works great for Timken tapered bearings. 

Customer of ours was swinging a sledge hammer driving a shaft out of a piece of equipment, was standing in a position where his legs where locked, swung and missed and the momentum of the sledge hammer twisted his body around and wrenched his back badly, he didn't realize he was really hurt just thought he wrenched it and needed to lie down and rest, a couple hours later he was paralyzed from the waist down and spends his days in a wheelchair.

Don't take a chance, go get checked out. 

I have a fluidamper on my 351/427, my engine is fully balanced with the exception of the pressure plate, I can spin it to 5500 in neutral and it runs really smooth but have always had the harmonic vibration on the hiway which I think has been there since installing the T5.

Try putting a short length of hose on the rear bowl vent, extend it as high as the air cleaner lid will allow, I think what is happening is under hard braking fuel is being forced up the vent and causes the engine to flood with raw fuel momentarily and die.

Friend of mine is on the list for one, or I should say he applied and filled out the questionaire. Not sure when he will find out if he gets to write that check. 

He already has a 07, 10, 13 GT500 and a 16 GT350, went for a ride in the GT350 yesterday, that 5.2 flat plane pulls like a freight train, he took it to 150 and it was starting to run out of gas at that point, sure not bad for a naturally aspirated engine. 
 

That plug is 3/8th's NPT, What size temp sender are you trying ti install ? Should be able to get a bushing to reduce to 1/4 or 1/8th inch NPT

I have a Center Force Dual Friction behind my 427 stroker, I will say that it is smooth and easy operating and holds great, but after several thousnad miles it developed a vibration that kept getting worse, I finally found that the centrifical weights were not staying centered on the diapraghm fingers, they would fly out until the large ring that limits their travel stopped them and then the whole set would move off to the side and throw the assy badly out of balance, I finally removed the weights and now the engine runs smooth again, a google search of this issue will find several threads on forums about this very issue. My next clutch won't be a Center Force but not sure what it will be yet, hopefully I don't need to be in the market any time soon. 

I have used TTY head bolts in large John Deere engines for many years and don't see anything wrong with them, they are designed to be stretched a given amount with the elasticity of the metal doing the job of maintaining a set clamp load on either the cylinder head or the connecting rods, the head bolts are rated to be used up to 5 times whereas the connecting rod bolts are used one time only. The problem with reusing the head bolts is that if you don't know the history of the engine you have no way of knowing how many times the bolts have been used. 

We had lots of trouble with head gaskets failing several years ago and the introduction of the TTY head bolts greatly helped with this issue. 

Check out Dakota Digital automotive accessories for their hidden antenna just for reference, I found an identical one on ebay that was much less expensive, I installed it to the windshield behind the rear view mirror, if you don't know it is there you never see it. while it isn't quite as good as the original antenna it does OK, but then I am pretty rural compared to some of you so in your case it might do very well. 

OK, I got the transmission back together and went for a test run last night, the vibration seems to be gone finally, I talked to Paul at the link that BobE posted and ordered the countershaft bearing stabilizer plate to better control the preload on the countershaft bearings, I did recheck how loose the countershaft was and it was .022 where it is supposed to be preloaded 2 -4 inch ponds of rolling torque,  the stamped steel bearing retainer was badly deformed causing this problem, So hopefully this problem is behind me now. 

Another vibration issue I had and found previouly was with the Center Force Dual Friction clutch, the fly weights on the diaphragm fingers were not staying centered under high RPM's because of the heavy ring around the weights the whole assy would move off center and vibrate like crazy, removing the weights resolved that problem, the next clutch will not be a Center Force but will cross that bridge when the time comes. 

Glen, what is spec for max runout ? On the machined surface for the rear bearing I measure .004 between centers on a lathe using a dial indicator.