I have a 77 F-250 highboy which i recently swapped out the c6 for an np435 i had a donor truck which was 2 wheel drive and was a 351m my truck is a 400 i replaced the clutch and everything else bolted right up. I also put in an electric fan from a taurus i then took the truck out on the road and i had serious vibrations couldnt get to 50mph without my teeth shaking loose i changed out the rear u-joints and now i can go 65 but i have a new vibration. When the truck is started the engine shakes really bad...you can visibly see the engine shaking quite badly but the engine runs good and idles well just rocks something feirce. The vibration starts at 40-45 and gets worse as you go up it goes waaaaaaaaaaaaaaaaaa stops for 2 seconds and then waaaaaaaaaaaa it keeps doing this until i let off the gas or put the truck in neutral....My guess is the flywheel for a 351m doesnt work for a 400 and i was under the assumption they were the same. Is there any other explanation for the vibration and motor shaking?

 

The flywheels are weighted the same.

 

Ok....so why else would my motor be shaking so much?

 

The same flywheel is used for both the 351M/400 = E5TZ-6375-H (updated Ford p/n)

When you swapped transmissions, did you line the bellhousing up to the two dowel pins (EAD-6397-B ~ 1/2" X 1") on the back of the engine?
If not lined up correctly, a vibration will result, which will eventually tear the center out of your flywheel.

 

Yes i know for a fact they are lined up due to fact it took my buddy and i awhile to get it installed and right...i thought motor mounts right away but they shouldnt just go to crap overnight the motor vibrates so bad that one of the exhaust clamps was hitting my front driveshaft.

 

Did you install the driveshaft correctly ?

 

Not sure if it would cause the vibration you have but you did not mention a pilot bearing being installed on your conversion

 

Bingo-

-Jared

 

It was actually the stupidest thing.....the passenger side motor mount was a touch loose as soon as i tightened it up the motor stopped shaking....but the vibration is still going on because the exhaust is now vibrating against the equalizer bar and the driveshaft still so ill have to take it to the exhaust guy next week and get that straigtened out....thanks for all the help even though that shouldve been the first thing i checked.